U.S. Defense Secretary Ash Carter says an American airstrike that may have killed a number of Iraqi soldiers seems to be "a mistake that involved both sides."

Carter has called Iraq's prime minister, Haider al-Abadi, to express condolences.

Carter spoke to reporters during a visit Saturday to the USS Kearsarge in the Persian Gulf.

The U.S. military says an airstrike Friday against Islamic State targets may have resulted in the death of Iraqi soldiers near the city of Fallujah. The military didn't say how many may have been killed. Other officials said the Iraqis initially reported that about 10 may have died.

The U.S. military says the strikes came in response to requests and information provided by Iraqi security forces on the ground near Fallujah, which IS controls.