The incident happened after coalition forces made up of US special ops troops and an Afghan solider were ambushed in the Zabul province.
When the group called for air support, the American troops were hit instead.
"Tragically, there is the possibility that fratricide may have been involved," a NATO statement said.
Fratricide is a military term for friendly fire.
An Afghan police chief told the Washington Post, “there were no Taliban casualties from the airstrike, but we had killed many of them during the day’s operation."
The New York Times reports this was one of the most deadly friendly fire events of the war.
The troops were carrying out security checks before the Afghan presidential election runoff this Saturday.