A U.S. official says a suspected insider attack by a member of Afghan security forces killed a U.S. soldier, bringing to 2,000 the number of U.S. military deaths in the 11-year-old conflict.
The U.S. official confirmed that the attack in eastern Wardak Province late on September 29 killed a U.S. soldier.
The International Security Assistance Force (ISAF) said a NATO soldier and a foreign civilian contractor were killed in the incident which also resulted in Afghan Army casualties.
ISAF commander, Lieutenant General Adrian Bradshaw, told journalists in Kabul: "Very sadly, one ISAF soldier and one ISAF civilian contractor were killed together with three Afghan National Army personnel. That is the reporting we have so far."
ISAF provided no further details on the incident but said a joint investigation was under way.
Provincial government spokesman Shahidullah Shahid said the shooting was sparked by an apparent "misunderstanding."
"Due to some misunderstanding an exchange of fire has taken place between ISAF and ANA [Afghan National Army] soldiers," Shahid said. "There are casualties but the reason behind the misunderstanding, firing, and exact number of casualties is not known yet. Both forces are investigating the matter."
He said the shooting occurred at an Afghan Army checkpoint in the Syedabad district, just outside a joint U.S.-Afghan base.
Afghan officials said at least two Afghan soldiers were killed in the shoot-out.
At least 52 members of the NATO-led force have been killed this year in attacks in which members of Afghan security forces turn their weapons on their Western allies.
Two weeks ago, ISAF restricted joint operations with its Afghan partners following a dramatic rise in such assaults.
But U.S. Defense Secretary Leon Panetta said on September 27 that most NATO-led troops have resumed joint operations with Afghan forces.
General John Allen, the top U.S. and NATO commander in Afghanistan -- in an interview with CBS's "60 Minutes" scheduled to be aired on September 30 -- said he was "mad as hell" about attacks by Afghan soldiers on Western troops.
Allen, according to excerpts from the interview released by the network, said "we're willing to sacrifice a lot for this campaign, but we're not willing to be murdered for it."
The Taliban routinely claim responsibility for insider attacks which they say were carried out by infiltrators.
NATO combat troops are set to withdraw from Afghanistan by the end of 2014, but a key part of the strategy is that coalition soldiers will remain to work alongside and train Afghan troops.