British soldiers who have served in Iraq and Afghanistan - particularly young men and those who have seen active combat - are more likely to commit violent crimes than their civilian counterparts, according to research published on Friday.

The study of almost 14,000 British soldiers deployed to Iraq and Afghanistan is the first to examine the link between military service and violent crime by using official criminal records.

Researchers said the findings could help military officials improve their risk assessment of violence among serving and ex-military personnel.

They stressed that although the study points to a serious problem for those affected, it does not mean all ex-soldiers will become violent criminals.

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"Just as with post traumatic stress disorder, this is not a common outcome in military populations," said Professor Simon Wessely, co-director of the Centre for Military Health Research at King's College London, who co-led the study.

"Overall you must remember that of those who serve in combat, 94 percent of those who come back will not offend."

The study found that those in combat roles were more than 50 percent more likely than those in non-combat roles to commit assaults or threaten violence after returning.

The problem was particularly striking among young men. Of around 3,000 soldiers aged under 30, more than 20 percent had a conviction for violent offences, compared with only 6.7 percent of civilian men in the same age group.

The study also highlights mental health problems in the military, and issues of alcohol abuse, post-traumatic stress disorder and aggressive behaviour, the researchers said.

Violent offending was most common among young men from the lower ranks of the army, said Deirdre MacManus from King's College London, who led the work and presented the results at a briefing in London.

This behaviour was strongly associated with a history of violent offending before joining the military, she said.

The study's publication, in the Lancet medical journal on Friday, comes as military chiefs in the United States say a soldier charged with slaying 16 civilians in Afghanistan last year should undergo a sanity review.

Anecdotal evidence and media coverage of violence and assaults committed by ex-servicemen has focused attention on whether serving in combat makes soldiers less stable and more prone to violent outbursts.

The study's results found that men who had seen combat in Iraq and Afghanistan were 53 percent more likely to commit violent offences than their fellow soldiers in non-combat roles.

Men who had multiple traumatic combat experiences had a 70 to 80 percent higher risk of becoming violent criminals.

David Forbes, an expert in post-traumatic mental health from the University of Melbourne, Australia, said this study showed for the first time the link between combat and interpersonal violence, and the need for better understanding of the mechanisms behind how combat enhances the risk of violence.

"By understanding these factors, we might develop more informed prevention and intervention programmes for troops as they reintegrate into civilian life," he wrote in a commentary.

Wessely said that having naturally higher levels of aggression was likely to be an attribute for many soldiers.

"Some people with aggressive dispositions make very good soldiers, that's the nature of the game," he said.

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Israeli troops on Friday shot dead three militants in an exchange of fire near the border with Egypt, an Israeli army spokeswoman said. 

An Israeli soldier was also killed, Israeli media reported, but the Israeli military declined immediate comment on that report.

"Three armed terrorists crossed the border into Israel and opened fire at troops securing workers who are building the border fence in the area ... They (attackers) were wearing flak jackets and were well-armed and carried explosive belts," spokeswoman Lieutenant-Colonel Avital Leibovich said.

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She said she did not have information on the identity or affiliation of the gunmen, but added that soldiers had "managed to thwart a major incident."

In June, militants crossed into Israel from Egypt's Sinai desert and fired on Israelis building a barrier on the border, killing a worker, before soldiers killed two of the attackers.

Israel is putting up the border fence to curb an influx of African migrants and improve security, hoping to complete it by the end of the year. It will run along most of the 165-mile frontier from Eilat, on the Red Sea, to the Gaza Strip. 

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Updated at 12:05 p.m. ET: Four U.S. soldiers fighting with the NATO-led alliance were killed in an apparent insider shooting in southern Afghanistan on Sunday, the Pentagon confirmed.

A Pentagon spokesman did not have further details, including which branch of services the Americans belonged to, Reuters reported.

The shooting took place in Zabol, a southern province where U.S. forces are based, and came a day after two British soldiers were shot dead by an Afghan policeman while returning from a patrol in southern Helmand province — a stronghold of the Taliban-led insurgency. 

Local authorities also told NBC News that the four killed were Americans.

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An International Security Assistance Force (ISAF) statement described the incident as "an insider attack suspected to involve members of the Afghan police."

What's leading Afghan troops to turn on coalition forces?

One attacker who was wearing an Afghan National Police uniform (ANP) was also killed in the fighting, a source told Reuters.

At least 51 foreign military personnel have been killed in "insider" attacks this year, attacks which have put a heavy strain on trust between the coalition and Afghanistan as they move towards handing security responsibility to Afghan forces by the end of 2014. 

Related:

• Two NATO soldiers killed by gunman thought to be Afghan police officer
• Deadly week for US in Afghanistan
• Two US Marines killed in attack on NATO base
• US suspends training for some Afghan recruits after 'insider' attacks

The rise in such attacks has led to the training of new recruits to the Afghan army and police being suspended. 

With foreign combat troops withdrawing from the increasingly unpopular and expensive war, the enormous cultural divide that still separates Afghans and their allies after 11 years of conflict has become more of a concern than ever. 

The NATO-led coalition and its Afghan counterparts have created a special Joint Casualties Assessment Team to investigate every attack, which number at least 37 this year. 

In more than half of cases, attackers are either killed or escape and the motive never emerges, making it more difficult for the coalition to stem the surge.

Adding to the toll of coalition deaths caused by insider attacks over the weekend, two were killed and nine wounded in Friday's attack on Camp Bastion, one of the worst attacks on a NATO-operated base all year. 

Six Harrier jets were destroyed and two were significantly damaged in the raid on the camp airfield, carried out by 15 insurgents wearing U.S. Army uniforms and split between three teams, a NATO statement said on Sunday.

U.S. suspends training for some Afghan recruits after 'insider' attacks

Three refueling stations were destroyed and six aircraft hangars were damaged. Britain's Prince Harry was at Camp Bastion at the time of Friday's attack, but was unharmed. 

All but one of the attackers were killed, with the remaining fighter taken into custody by coalition forces. 

ISAF said the attack was "well-coordinated". A statement said: "The insurgents, organized into three teams, penetrated at one point of the perimeter fence. The insurgents appeared to be well equipped, trained and rehearsed. Dressed in U.S. Army uniforms and armed with automatic rifles, rocket propelled grenade launchers and suicide vests, the insurgents attacked coalition fixed and rotary wing aircraft parked on the flight line, aircraft hangars and other buildings."

With foreign combat troops withdrawing from the  unpopular and expensive war by the end of 2014, the enormous cultural divide that still separates Afghans and their allies after 11 years of war has become more of a concern than ever.

The NATO-led coalition and its Afghan counterparts have created a special Joint Casualties Assessment Team to investigate every attack.

In more than half of the cases, attackers are either killed or escape and the motive for the incident never emerges, making it more difficult for the coalition to stem the surge in incidents.

Reuters contributed to this report.

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Updated at 3:08 p.m. ET: Seven American troops were killed Thursday when the Black Hawk helicopter they were traveling in crashed in southern Afghanistan, possibly shot down by enemy forces, U.S. officials told NBC News.

Three of the Americans were U.S. Navy sailors – two were Navy SEALS and one was an explosive ordnance disposal sailor.

Three Afghan soldiers and an Afghan interpreter were also killed, bringing the total number of dead to 11, officials said.

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According to U.S. officials, the helicopter had just landed in a "hot zone" to insert a small number of combat forces to support an ongoing operation. It was apparently shot down shortly after it took off to depart the area.

The helicopter was totally destroyed on impact.

U.S. officials at the Pentagon and in Kabul tell NBC News that American forces engaged in a shootout with enemy forces on the ground who were trying to reach the crash site.  

The crash marked another deadly day for the U.S. in Afghanistan, less than a week after six American service members were gunned down, apparently by two members of the Afghan security forces they were training to take over the fight against the insurgency as international combat troops prepare to exit the country by the end of 2014.

The recent spike in American deaths and attacks by Afghan allies have stirred fresh doubts about the prospects for the U.S. plan to leave a capable Afghan government in place when most troops depart after more than a decade of war.

Spokesman Brig. Gen Gunter Katz said the NATO coalition is investigating the cause of Thursday's crash in Shah Wali Kot, Kandahar.

Dozens killed in Afghan suicide bomber attacks

The Taliban claimed responsibility for shooting down the helicopter, saying they were able to bring down the aircraft during a raid by the "invaders."

While the Taliban often exaggerate their victories and are quick to claim any incident involving the death of foreign troops, it could be significant that they took responsibility for Thursday's crash hours before NATO announced it.

"Nobody survived this," Taliban spokesman Qari Yousef Ahmadi told The Associated Press by telephone.

Insurgent hotbed
The area where the helicopter went down is an insurgent hotbed and supply route, lying north of Kandahar city near Zabul and Uruzgan provinces. The insurgents regularly attack police checkpoints around the rural villages of the district and plant bombs in the road to catch passing government vehicles.

Is the Taliban softening its stance on girls' education?

Thursday's crash is the deadliest since a Turkish helicopter crashed into a house near the Afghan capital, Kabul, on March 16, killing 12 Turkish soldiers on board and four Afghan civilians on the ground, officials said.

The Taliban shot down a CH-47 Chinook transport helicopter in August 2011, killing all 38 people on board, including 25 U.S. special operations service members.

The Sikorsky UH-60 Black Hawk is a medium-lift helicopter that has served as the U.S. Army's workhorse since the 1980s.

 

The U.S.-led NATO force in Afghanistan has relied heavily on utility helicopters such as the Black Hawk to ferry troops, dignitaries and supplies around the mountainous terrain, thus avoiding the threat of ambushes and roadside bombs.

Afghan bomber kills senior Army leader, 2 majors

Afghanistan's Interior Ministry said this week that the Taliban had not let up on attacks during Ramadan and security forces had stepped up security ahead of the Eid al-Fitr festival, which ends Islam's holiest month. 

A half-yearly report by the United Nations last week said 1,145 civilians have been killed between Jan. 1 and June 30 this year as well as 1,954 wounded, representing a 15 percent decline on last year due to a severe winter that hampered fighting.

Homemade bombs and suicide attacks remain the biggest killers of Afghan civilians and Afghan and foreign troops. 

Reuters and The Associated Press contributed to this report.

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Updated at 9:25 a.m. ET: KABUL, Afghanistan -- Three U.S. Marines were shot dead by an Afghan worker on a military base in southern Afghanistan, an Afghan official told NBC News, raising to six the number of American service members who died in rogue attacks in the country in 24 hours.

The shooting took place on Friday night in the Garmsir district of Helmand province, not far from where three U.S. Marines were killed by an Afghan gunman earlier in the day, Dawood Ahmadi, spokesman for the Helmand province governor, told NBC News' Atia Abawi on Saturday.

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In the earlier incident, an Afghan police commander opened fire on the U.S. service members after inviting them to a meeting to discuss security, according to Reuters. Another service member was injured in that attack.

Three US special ops troops killed, Afghan officials say

"Let me clearly say that those two incidents clearly do not reflect the overall situation here in Afghanistan," chief ISAF spokesman Brigadier-General Gunter Katz told journalists on Saturday.

Reuters reported that the Marines killed in the latest attack were shot dead by a base employee who turned his gun on them. Military sources said the man had not been wearing a uniform and it was unclear how he got hold of the weapon.

"We are still investigating the incident to find out about the shooter and who he actually is -- whether an Afghan soldier or a civilian," Ahmadi, the Helmand governor's spokesman, told NBC News.

In a statement, ISAF said the gunman had been detained.

On Saturday, Afghanistan's President Hamid Karzai condemned the killings of all six U.S. Marines in the two incidents, calling the perpetrators of the attacks "terrorists." He added that the country's enemies did not want "a secure Afghanistan with a stable army."

Bloody week
Green on blue shootings, in which Afghan police or soldiers turn their guns on their Western colleagues, have seriously eroded trust between the allies as NATO combat soldiers prepare to hand over to Afghan forces by 2014, after which most foreign forces will leave the country. 

The NATO force says there have been 26 such attacks on foreign troops since January in which 34 people have been killed. Last year, there were 21 attacks in which 35 people were killed.

But a coalition spokesman said the killings by the Afghan worker would not be included in that tally as it did not involve a member of the Afghan security forces.

However Katz said the incidents were relatively isolated and were not hurting morale or cooperation between foreign forces and the 350,000-strong Afghan Security Forces.  

"We have almost 500,000 police and soldiers working together, side by side, enhancing their trust and enhancing their cooperation in order together to fight for a better future for this country," he said. 

Still, it was a bloody week for NATO forces in Afghanistan. Earlier on Friday, the Pentagon confirmed that three U.S. service members -- including a senior Army leader -- and an American aid worker were killed Wednesday by a suicide bomber in Kunar province.

The victims included Command Sgt. Maj. Kevin J. Griffin, the most senior enlisted soldier for the 4th Brigade Combat Team, 4th Infantry Division. Griffin, 45, of Riverton, Wyo., was a Bronze Star recipient who first enlisted in the Army in 1988.

Maj. Thomas E. Kennedy, 35, of West Point, N.Y., and Air Force Maj. Walter D. Gray, 38, of Conyers, Ga., were also killed. USAID foreign service officer Ragaei Abdelfattah was identified as the other victim.

On Tuesday, two gunmen wearing Afghan army uniforms killed a U.S. soldier and wounded two others in Paktia province in the east.

Violence in Afghanistan is at its fiercest since U.S.-led Afghan troops overthrew the Taliban government in 2001. Insurgents have extended their reach from traditional strongholds in southern and eastern areas to parts of the country once considered safe.

This is a breaking news story. Check again for more updates.

NBC News' Atia Abawi, Fazl Ahad and Reuters contributed to this report.

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News commentary

Those who have been in combat will testify to the catastrophic insults to the body that modern weapons can inflict. War is horrifying, and nothing can prepare the novice for the destruction that it can cause. Nor do we easily get used to the images of it, and they stay with us forever.

Recently released by the Los Angeles Times, the grisly photos of soldiers posing with the remains of dead Taliban fighters  have raised a variety of observations: From the notion that they are similar to the harmless pranks of adolescents to the assessment that their publication will be a catastrophe for the American mission in Afghanistan.

As with most extremes, neither is the case. We should also reject the argument that this incident, the burning of Korans and the deliberate murder of women and children, such as those allegedly carried out by Staff Sgt. Robert Bales, are all the same. 

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No excuses
Here are the facts: The pictures are about two years old and were of Taliban fighters killed when a bomb they were putting into position detonated prematurely. The photos were sent to the Times by someone who said he wanted to highlight the threat to our troops caused by the poor leadership of the unit, a part of the 82nd Airborne Division.

But, although the Times suggested that the concern was merely inadequate physical security rather than a climate of generally weak discipline, it is the latter issue that is the most striking.

When the Times notified the Defense Department that it had the photos, the Pentagon asked the paper not to publish them, arguing that they would incite the enemy to attack Americans. The Times responded that it had an obligation to publish them, citing their readers' right to be informed.

In my view, both the Defense Department and the newspaper are full of baloney: The Taliban don't need any encouragement to attack us, and a big part of the motivation of the Los Angeles Times is to sell newspapers.

More nuanced has been comment from some quarters that the troops, who were mugging for the camera, were letting off the steam that accumulates under the duress of war; that their actions were in response to having lost buddies to the mindless ferocity of the Taliban.

While these are understandable reasons, they are not excuses, of course, and the paratroopers' actions were publicly decried by government officials. Many cited long-standing rules, promulgated after similarly embarrassing episodes, stating that such antics are impermissible.

Lack of leadership
But the truth is that you can't merely legislate against dumb behavior. In and out of combat, good units get that way because they are well led.

Poor leadership can create poor units in a very short period of time, particularly under stress. While good leadership can bring any organization through the most horrendous circumstances with only physical scars.

The leadership of the brigade in the 82nd that is at the center of this photo controversy was evidently already known as weak by the chain-of-command above it. There are many military organizations that have endured more harrowing circumstances with less damage to discipline.

It is not easy being a leader in uniform, but there is a responsibility attached to it that is found nowhere else in society. Military service is a sacrifice and those who volunteer for it are our patriots. But service is no game, and because so much is at stake, standards of deportment must be extremely high.

We are frequently reminded of it, but it bears repeating nonetheless: a commander is responsible for everything that happens or fails to happen in his unit, and it is he who sets the standards in his organization. Accepting less than professional behavior will minimize the service and sacrifice of those who have taken seriously their responsibilities as the guardians of our freedom.

Col. Jack Jacobs was awarded the Medal of Honor for “conspicuous gallantry and intrepidity in action at the risk of his life above and beyond the call of duty” in the battle he describes above. His first assignment in the Army, in 1966-1967, was in Company C, 2nd Battalion (Airborne) 505th Infantry of the 82nd Airborne Division, the same division as the troops in this incident.

Click here to read the complete Medal of Honor citation. 

He is the author of a memoir: “If Not Now, When? Duty and Sacrifice in America’s Time of Need”