Five coalition troops were killed in an apparent incident of friendly fire in southern Afghanistan, according to reports from coalition headquarters in Kabul Tuesday.

NBC News Chief Foreign Correspondent Richard Engel says that the five U.S. troops that were killed were special operations forces. In a tweet, Engel said "According to U.S. officials, a B-1 stealth bomber unleashed the strike which went wrong and struck the American Forces."

The incident took place Monday when the unit came into contact with enemy forces, the coalition said in a statement. The incident is under investigation.

"Tragically, there is the possibility that fratricide may have been involved," the statement said.

The incident comes as U.S. and allied forces are drawing down and turning the fight over to Afghan security forces.

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.

A senior police official in southern Zabul province said the coalition soldiers may have been killed when they called in 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 troops came under attack from the Taliban and called in air support.

Rooghlawanay's account could not be confirmed immediately.

The deaths bring to 36 the number of NATO soldiers killed so far this year in Afghanistan, with eight service members killed in June.

Contributing: The Associated Press