Two U.S. service members were killed and five injured during combat operations in northern Iraq on Sunday, the military said, adding that initial reports indicated the incident was not due to enemy contact.
In a statement the military said the incident was under investigation. The service members were part of a U.S.-led coalition fighting the Islamic State in Iraq.
"The entire counter-ISIS Coalition sends our deepest condolences to these heroes' families, friends and teammates," said Lieutenant General Stephen Townsend, commander of the Combined Joint Task Force - Operation Inherent Resolve.
"I hope there is some small solace in knowing their loss has meaning for our country and all the nations of the Coalition as the fallen service members were fighting to defeat a truly evil enemy and to protect our homelands."
Islamic State group claimed responsbility on the attack.
Islamic State fighters fired Grad rockets on American troops east of Tal Afar, a town still under control of the militants west of Mosul, according to an online statement from the group.
Islamic State group's self-proclaimed "caliphate" effectively collapsed last month, when U.S.-backed Iraqi forces completed the recapture of Mosul, the militants' capital in Iraq, after a nine-month campaign