A senior defense official confirms to Fox News this morning that five U.S. Special Forces were killed Monday in what appears to have been a friendly fire incident in southern Afghanistan.

According to multiple reports, a NATO aircraft unleashed an airstrike on the wrong position after coalition forces encountered the enemy, and air support was called in.

At least one Afghan officer was also killed.

Read more from FoxNews.com below and stay tuned to Fox News for the latest updates on this developing story.

"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 soldiers 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 air strike," Rooghlawanay said.