U.S. Defense Secretary Ash Carter said an American airstrike that killed up to 10 Iraqi security forces appears to have been "a mistake that involved both sides."
Carter said a missile fired by a U.S. warplane hit Iraqi security forces near the city of Fallujah in central Iraq, which is controlled by Islamic State fighters.
Iraq's Defense Ministry said about 10 soldiers died in the incident on Friday.
The U.S. military said targeting information for the airstrike came from Iraqi security forces on the ground near Fallujah.
Speaking to reporters Saturday aboard the U.S. amphibious assault ship the USS Kearsarge in the Gulf, Carter said he called Iraqi Prime Minister Haider al-Abadi to express his condolences, and that they agreed the "regrettable" incident would be investigated.
Carter said, "These kinds of things happen when you're fighting side by side," as Iraqi and American forces are. But he also said he and the Iraqi prime minister said the campaign to expel Islamic State forces from Iraqi territory will move forward and intensify.
"It has all indications of being a mistake of the kind that can happen on a dynamic battlefield between two forces, ours and Iraqi forces, that are working so closely together," Carter said during his visit to the American warship. "An accident happened. It's tragic. I expressed our condolences. But [Abadi] and I both recognize that things like this can happen in war."
Carter is on a trip through the Middle East and Afghanistan.
He met with Abadi a few days ago during a stop in Baghdad to assess the state of the allied campaign against Islamic State extremists.