A man wearing an Afghan army uniform shot dead a US service member in the east of the country, one of two NATO troops killed today, military officials said.
The Taliban took credit for the attack.
It was the 15th incident this year in which Afghan soldiers or insurgents disguised in military uniforms have turned their weapons on foreign troops.
The shootings have increased the level of mistrust between the US-led coalition and its Afghan partners and raised questions about the readiness of local forces to take over from NATO ahead of a 2014 deadline for the withdrawal of foreign combat troops.
An Afghan defence official said the incident took place in Kunar province in northeast Afghanistan. The official spoke on condition of anonymity because he was not authorised to brief the media.
NATO did not disclose the nationality of the trooper killed, but a Pentagon spokesman, Navy Capt John Kirby, said the service member was an American.
Kirby said US and allied officials are troubled that these attacks are continuing with some regularity despite efforts to improve the vetting of Afghans who are recruited into the army and police.
"It continues to be a very worrisome issue for us and for our Afghan partners," Kirby said. The coalition said an investigation into today's attack was under way. In a statement, Taliban spokesman Zabiullah Mujahid claimed the insurgent group was behind the shooting.
The Taliban regularly take credit for attacks in the country, even if they were not involved.