Five U.S. troops who are Special Operations Forces were killed in a possible friendly-fire incident in southern Afghanistan on Monday, Defense Department officials say.
A senior U.S. defense official told Fox News early Tuesday that the troops were killed when they requested air support from a B-1 bomber after coming in contact with enemy forces.
"Five American troops were killed yesterday during a security operation in southern Afghanistan. Investigators are looking into the likelihood that friendly fire was the cause. Our thoughts and prayers are with the families of these fallen," Pentagon spokesman Rear Adm. John Kirby said.
Earlier Tuesday, a statement from NATO confirmed the service members' deaths, but did not give further details on their nationalities. Coalition policy is for home countries to identify their military dead.
"The casualties occurred during a security operation when their unit came into contact with enemy forces," the statement from NATO's International Security Assistance Force read. "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 friendly fire during the nearly 14-year Afghan war. 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 the southern Kandahar province.
A senior police official in southern Zabul province said the coalition troops may have been killed when they called in for close air support.
Provincial police chief Gen. Ghulam Sakhi Rooghlawanay said Afghan and NATO troops conducting a joint operation in the area's Arghandab district early Monday came under fire by the Taliban.
"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 airstrike," 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 only U.S. troops now involved in combat operations are usually Special Operations Forces that mentor their Afghan counterparts. They often come under fire and are responsible for calling in air support when needed. Because of constraints placed by outgoing Afghan President Hamid Karzai, such airstrikes are usually called "in extremis," or when troops fear they are about to be killed.
Karzai blamed a similar airstrike called in by special forces mentoring an Afghan operation for killing a dozen civilians during an operation in northern Parwan province. The U.S. military vehemently denied the charge, saying that two civilians were killed in crossfire with Taliban militants and that airstrike was called in when forces thought they were about to be killed by insurgents.
The Taliban claimed responsibility for the attack against the joint force in Zabul.
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.
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.
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.
Fox News' Jennifer Griffin and The Associated Press contributed to this report.