Five NATO troops -- all Americans -- were killed during an attack in southern Afghanistan, NATO said Tuesday, and friendly fire may be to blame for their deaths.

An Afghan official told CNN on Tuesday that the Americans were killed Monday due to friendly fire, telling the network that a NATO jet called in to assist in quelling a Taliban attack actually bombed its own soldiers instead of Taliban fighters. But NATO was not as definitive.

“Tragically, there is the possibility that fratricide may have been involved,” the alliance said, in a statement.

NATO did not give further details of the attack, according to Associated Press. If friendly fire is found to have been the cause of the deaths of the five Americans, it would be one of the worst coalition-on-coalition incidents in the war in Afghanistan.

Ghulam Sakhi Roghliwanai, police chief for Zabul province, said the NATO troops came under rocket fire from Taliban militants after conducting joint military sweeps and that's when the NATO troops called in air support.

“But the airstrike mistakenly bombed their own friends, too,” Roghliwanai told CNN.