A man wearing an Afghan army uniform has shot dead a US soldier, one of two Nato service members killed today.
The Taliban claimed responsibility for the attack.
It was the 20th incident this year in which Afghan soldiers or insurgents disguised in military uniforms have turned their weapons on foreign troops.
The incidents have raised 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.
The incident took place in Kunar province in north-east Afghanistan. Nato did not disclose the nationality of the trooper killed, but a senior US defence official said the victim was American. An investigation is under way.
Taliban spokesman Zabiullah Mujahid claimed the insurgent group was behind the attack. The Taliban regularly take credit for attacks, even if they were not involved.
The threat from Afghan soldiers or militants disguised in uniforms turning on Nato troops has grown more deadly. The 20 insider attacks so far this year have killed 13 soldiers, compared to 21 attacks last year that killed 35 coalition service members.
That compares with 11 fatal attacks and 20 deaths in 2010. And in 2007 and 2008 there were a combined total of four attacks and four deaths.
The coalition routinely reports each time an American or other foreign soldier is killed by an Afghan in uniform, but the military is underreporting the number of overall attacks.
It does not report attacks in which an Afghan wounds – or misses – his US or allied target. It also does not report the wounding of troops who were attacked alongside those who were killed.
US officials say that in most cases the rogue soldiers are motivated not by sympathy for the Taliban or on orders from the insurgents, but rather act as a result of personal grievances against the coalition.
Nato said a second service member died following an insurgent attack in southern Afghanistan, but did not provide further details.