Three U.S. soldiers were killed by a man wearing an Afghan uniform in southern Afghanistan's Helmand Province.
Officials in Helmand said three U.S. Special Forces soldiers were killed by an Afghan police officer who had invited them for a meal.
The U.S. military in Afghanistan confirmed that three soldiers had been killed by "an individual in an Afghan uniform" in Helmand's Sangin district late on August 9, but gave no further details.
In Kabul, NATO spokesman Brigadier General Gunter Katz said that the alliance was investigating the killings.
"I cannot give you the specific details for this investigation since it is still ongoing," Katz said.
"What I can tell you, however, is that when we are looking to other [previous] incidents, we take them very seriously, they are very tragic, but they do not reflect the overall situation here in Afghanistan."
Another 'Green-On-Blue' Attack
Afghan sources told AFP that the soldiers were shot dead by a police checkpoint commander in the Sangin district who had invited them for a predawn Ramadan breakfast meeting early on August 10.
Local officials said the commander, Asadullah, escaped. A purported Taliban spokesman, Qari Yusuf Ahmadi, told AFP that Asadullah joined their ranks after the killings.
The Taliban regularly exaggerate attacks or claim credit for killing foreign soldiers even if they are not involved.
An increasing number of Afghan soldiers and police have turned their weapons against NATO colleagues helping them to fight a decade-long insurgency by the Taliban Islamists, who were ousted from power in a U.S.-led invasion in 2001.
These so-called green-on-blue shootings have seriously eroded trust between the allies as NATO combat troops prepare to hand over to Afghan forces by 2014.
According to NATO, there have been 24 such attacks on foreign troops since January in which 28 people have been killed.
Latest Civilian Casualties
Meanwhile, six Afghan civilians were killed and another was injured when their vehicle hit a roadside bomb in Helmand Province on August 10. Authorities said Taliban insurgents had planted the bomb in the Musa Qala district.
Roadside bombs are a favorite weapon of Taliban militants fighting government forces and their NATO backers. But they often miss military targets and kill civilians.
The latest deaths come two days after a report by the United Nations said more than 3,000 civilians have been killed and wounded in the first six months of this year.
The UN blamed 80 percent of the deaths on insurgents, saying more than half were caused by roadside bombs.
With reporting by Reuters, AFP, AP, and BBC