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.

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.

Slideshow: Afghanistan: Nation at a crossroads

Ahmad Jamshid / AP

More than ten years after the beginning of the war, Afghanistan faces external pressure to reform as well as ongoing internal conflicts.

Launch slideshow

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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Updated at 12:35 p.m. ET: KABUL, Afghanistan -- A man wearing an Afghan army uniform shot and killed three American Marines, the U.S. military command said Friday. Afghan officials said the victims were American special operations forces troops.

Reuters reported that an Afghan police commander opened fire on the service members after inviting them to a meeting to discuss security. A U.S. military official confirmed the three deaths and said another service member had been injured during the incident.

NBC News' Jim Miklaszewski reported that a "lone gunman" remained on the loose and was being hunted. U.S. military officials said all of the American victims were Marines.

Citing Afghan officials, Reuters said the American special operations forces members were killed late Thursday while attending a meeting in the Sarwan Qala area, in what appeared to be a planned attack by rogue Afghan forces.

"The commander was Afghan National Police in charge of local police in Sangin," a senior Afghan official told Reuters on condition of anonymity. Sangin is a district of Helmand province.

"It looks like he had drawn up a plan to kill them previously," the official added.

A military official told NBC News' Courtney Kube that it was unclear whether the gunman was a member of the Afghan security forces or whether he was just wearing a uniform.

Taliban spokesman Qari Yousef Ahmadi told The Associated Press by telephone that the attacker, whom he identified as a member of Helmand police named Asadullah, had been helping U.S. forces train Afghan local police troops. However, the Taliban has made false claims about the details of attacks in the past.

The attack is the third killing this week of coalition soldiers by Afghans who are training to take over responsibility for security once most international forces leave in 2014.

So-called "green on blue" shootings, in which Afghan police or soldiers turn their guns on their Western mentors, have seriously eroded trust between the allies.

According to NATO, there have been 24 such attacks on foreign troops since January in which 28 people have been killed. Last year, there were 21 attacks in which 35 people were killed.

Senior Army leader slain
Earlier, 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.

And on Thursday, two Afghan soldiers tried to gun down a group of NATO troops outside a military base in eastern Afghanistan. No international forces were killed, but one of the attackers was killed as NATO forces shot back.

NBC News' Courtney Kube, Jim Miklaszewski and Atia Abawi, The Associated Press and Reuters contributed to this report.

Slideshow: Afghanistan: Nation at a crossroads

Ahmad Jamshid / AP

More than ten years after the beginning of the war, Afghanistan faces external pressure to reform as well as ongoing internal conflicts.

Launch slideshow

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Abdul Khaleq / AP

An Afghan policeman checks a man entering the police station near the gate of a joint civilian-military base where two British soldiers part of the NATO forces were killed south of Kabul in Afghanistan.

Updated at 12:26 p.m. ET: Three NATO troops were killed in Afghanistan on Monday in two separate attacks apparently carried out by members of local security forces.

A gunman wearing an Afghan army uniform killed two NATO troops in southern Afghanistan, while another was shot in eastern Afghanistan by an alleged member of the Afghan Local Police.

The deaths appeared to be the latest in a string of so-called "green on blue" attacks in which Afghan security forces have turned on their international colleagues or mentors.

The incident in which two died took place in Lashkar Gah in the southern Helmand province, an Afghan security source told Reuters. It will be investigated by a joint Afghan-NATO team.

"The individual who opened fire was killed when coalition forces returned fire," NATO's International Security Assistance Force (ISAF) said in a statement about that incident. It did not identify the two NATO soldiers or give their nationality.

The Lashkar Gah base is dominated by British forces and a NATO official told The Associated Press the two victims were British soldiers. The official spoke on condition of anonymity because the information had not yet been formally announced.

Britain's Ministry of Defense said it was aware of the Lashkar Gah incident, but refused to release any more details or confirm whether the soldiers were British.

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In a later statement about the shooting in eastern Afghanistan, ISAF said: :According to operational reports, the ISAF service member was shot by an alleged member of the Afghan Local Police as the security force approached an ALP checkpoint. The circumstances behind the incident are currently under investigation by Afghan and coalition forces."

There were no further details immediately available.

Insider attacks have mounted in recent months as tension between Afghanistan and its foreign backers rises over a series of incidents including the burning of Qurans at a NATO base and the killing of 17 villagers in southern Kandahar for which a U.S. soldier has been charged.

Before Monday's attack, 13 members of the NATO-led force had been killed this year in what appeared to be attacks by members of Afghan forces, the commander of U.S. and NATO forces, General John Allen, told a U.S. Senate committee last week.

About 70 members of the NATO force have been killed in 42 insider attacks from May 2007 to January this year.

The shootings raise new concern about the reliability of Afghan forces and their ability to take over security responsibilities by the end of 2014, when most Western combat forces leave.

Reuters, The Associated Press, NBC News' Cheryll Simpson and msnbc.com staff contributed to this report.

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KABUL, Afghanistan - A roadside mine killed 19 civilians, including children, and injured another five when it exploded in the southern Afghan province of Helmand on Wednesday, the provincial government told Reuters.

The strike on the minibus came a day after at least 59 people were killed in sectarian attacks in three cities across the country, and refocused attention on the fragile Afghan security situation.

•  'Grim new precedent': Dozens die as sectarian violence erupts in Kabul

The vehicle was driving on a road in Helmand province's volatile Sangin district — a Taliban stronghold — when it hit the bomb, said Daoud Ahmadi, a spokesman for the Helmand government, according to The Associated Press.

After Tuesday's attacks, the largest of which targeted a Shiite Muslim shrine in the capital Kabul, Afghan President Hamid Karzai cancelled a planned visit to Britain to return straight home, wiping out any residual optimism from an international conference about the future of Afghanistan, held on Monday in Germany.

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At least five children were among the dead in the Helmand attack, Ahmadi said. No one immediately claimed responsibility for the attack — a common situation when bombs kill civilians.

On Tuesday, twin bombings on Shiite Muslims celebrating the holiday of Ashoura sparked fears that attacks in Afghanistan might be taking on a sectarian dimension for the first time. Ashoura honors the death of Imam Hussein, a grandson of the Prophet Muhammad, in 680 A.D.

A suicide bomber slaughtered dozens of Shiite worshippers and wounded more than 160 others Tuesday outside a Kabul shrine where hundreds had gathered to worship.

One U.S. citizen was also among the dead, according to a statement issued by the American embassy in Kabul. The deceased was not a government employee, U.S. Embassy spokeswoman Megan Ellis said, but declined to give further details.

The NATO-led International Security Assistance Force (ISAF), responsible for security across much of the country, says it is winning the war against the Taliban.

But if Tuesday's bombing sets a precedent for violence between the Sunni Muslim majority and the Shiite minority, it would severely stretch army and police resources.

• Emotional scenes as funerals are held for victims of Afghan sectarian attacks

At a funeral ceremony on Wednesday for victims of the attack, hundreds of Shiite Muslims bore aloft the bodies of the dead, chanting that because they had been killed at a Muslim ceremony, they had died in the name of the Prophet Muhammad.

"We were sacrificed for you," they shouted. "Where is the government, where are the members of parliament? Why they don't join our mourning? It creates a gap between people and the government," said Muhammad, 40 years old, who said one his relatives died in the Kabul blast.

The Associated Press and Reuters contributed to this report.

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