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.

More world stories from NBC News:

• 'There will be no winner in Syria,' UN chief warns
• Three US special ops troops killed, Afghan officials say
• Body found at home of missing UK girl's grandmother
• Day at Olympics well worth $1,000 for family of four, NJ fans say
• Notorious Colombian druglord arrested, headed to US for trial
• Who'll win the gold medal for partying? Olympians let hair down
• One year after London riots, a family still grapples with fallout
• Video: This $4,000-per-jar caviar boasts socialist roots
• Are these German protesters the world's oldest squatters?