Five Americans troops were killed in an apparent friendly fire incident in southern Afghanistan, a U.S. defense official said Tuesday, in one of the worst such incidents involving United States or coalition troops since the start of the nearly 14 year war.

The official insisted on anonymity because he was not authorized to speak publicly to journalists.

The U.S.-led international coalition said the service members were killed in an apparent friendly fire incident, which an Afghan official said took place in southern Zabul province. A statement said all five soldiers died on Monday but did not give further details on the attack or the nationality of the soldiers.

If confirmed, it would be one of the most serious cases involving coalition-on-coalition friendly fire during the war.

``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,'' the coalition said in an announcement.

One of the worst came in April 2002 when four Canadian soldiers were killed when an American F-16 jet fighter dropped a bomb on a group near a night firing exercise in southern Kandahar.

A senior police official in southern Zabul said the coalition soldiers may have been killed when they called for close air support.

Provincial police chief Gen. Ghulam Sakhi Rooghlawanay said there was a joint operation by Afghan and NATO troops in the area's Arghandab district early Monday. After that operation was over, the troops came under attack from the Taliban and called in air support, he said.

“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,'' Rooghlawanay said.

There was no way to independently confirm Rooghlawanay's comments. The coalition would not comment and NATO headquarters in Brussels also declined to comment.

The Taliban claimed responsibility for the attack against the joint force.

The insurgents have intensified attacks on Afghan and foreign forces ahead of the country's presidential election runoff 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 on Monday night between foreign troops and Taliban fighters in the Arghandab district. Ahmadi claimed a “huge number” of NATO soldiers were killed or wounded in the fighting. The Taliban often exaggerate their claims.

Separately, a NATO statement said a service member died Monday 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 service members killed in June.

Casualties have been falling in the U.S.-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. (AP)