Five U.S. service members and at least one Afghan soldier died Monday, when a coalition aircraft accidentally unleashed an airstrike on their position, The New York Times reported Tuesday.
NATO said the allied forces had been fighting Taliban militants when it happened.
“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,” NATO’s International Security Assistance Force (ISAF) said in a statement Tuesday morning.
White House spokesman Josh Earnest said the "thoughts and prayers" of the White House were "with the families of those who were killed in Afghanistan earlier today."
"Our hearts here at the White House are heavy as a result of this loss," he said.
While Earnest confirmed President Obama had been briefed on the incident, he would not confirm reports that troops died due to a coalition airstrike.
"The Department of Defense is still looking into what exactly happened, and it will be their responsibility to determine, you know, what actually led to their death," he said. "So I'm going to hold off on weighing in any further pending that investigation of what exactly happened."
The U.S. soldiers and their Afghan counterparts came under attack and requested air support, CBS News reported.
A Taliban spokesman, Qari Yousef Ahmadi, said foreign troops and Taliban fighters fought in Arghandab district in southern Zabul province, CBS said.
The province’s Afghan police chief told CBS that the requested aircraft mistakenly bombed the U.S. and Afghan forces on the ground.
The news comes about two weeks after President Obama announced the U.S. withdrawal plan from Afghanistan. A majority of forces will leave by the end of the year and 9,800 will remain through 2016.
This story was updated at 3:00 p.m.