Five Nato service members have been killed in an apparent friendly fire incident in southern Afghanistan, the international coalition says.
A US defence official said the five were American.
A Nato statement said: "The casualties occurred during a security operation when their unit came into contact with enemy forces. Tragically, there is the possibility that fratricide may have been involved. The incident is under investigation.
"Our thoughts are with the families of those killed during this difficult time."
If confirmed, it would be one of the most serious cases involving coalition-on -coalition fire during the nearly 14-year Afghan conflict. One of the worst came in April 2002 when four Canadian soldiers were killed when an American F-16 dropped a bomb on them near a night firing exercise in southern Kandahar.
A senior police official in southern Zabul province said the soldiers in the latest incident might have been killed when they called for close air support.
Provincial police chief General Ghulam Sakhi Rooghlawanay said: "There was a joint operation by the joint Afghan and foreign forces in Arghandab district of Zabul province on Monday. After the operation was over on the way back, the joint forces came under the attack of insurgents, then foreign forces called for an air support. Unfortunately five Nato soldiers and one Afghan army officer were killed mistakenly by Nato air strike."
The Taliban claimed responsibility for the attack. The insurgents have intensified attacks on Afghan and foreign forces ahead of the country's presidential election run-off on Saturday. Officials are concerned there could be more violence around the time of the vote, although the first round in April passed relatively peacefully.
A Taliban spokesman, Qari Yousef Ahmadi, said a battle took place between foreign troops and Taliban fighters in the Arghandab district. He claimed a "huge number" of Nato soldiers were killed or wounded in the fighting but the Taliban often exaggerates its claims.
Separately, a Nato statement said a service member died yesterday as a result of a non-battle injury in eastern Afghanistan.
The deaths bring to 36 the number of Nato soldiers killed so far this year in Afghanistan, with eight killed in June.
Casualties have been falling in the US-led military coalition as its forces pull back to allow the Afghan army and police to fight the Taliban insurgency. All combat troops are scheduled to be withdrawn from the country by the end of this year.