The American airstrike that may have killed a number of Iraqi soldiers on Friday seems to be "a mistake that involved both sides," U.S. Defense Secretary Ash Carter said Saturday. He called Iraqi Prime Minister Haider al-Abadi to express condolences.

Speaking to reporters during a visit to the USS Kearsarge in the Persian Gulf, Carter said the incident near the western Iraqi city of Fallujah was "regrettable."

"These kinds of things happen when you're fighting side by side as we are," Carter said. He said the airstrike Friday "has all the indications of being a mistake of the kind that can happen on a dynamic battlefield."

Carter, who spent two days in Iraq this past week, called Abadi from the USS Kearsarge, an amphibious assault ship supporting coalition missions in Iraq and Syria against Islamic State militants. The Kearsarge carries a Marine expeditionary unit and naval aircraft.

The Pentagon chief did not provide details about the airstrike, which the U.S. military headquarters in charge of the war effort in Syria and Iraq said was one of several it conducted Friday against IS targets. A U.S. military statement said the airstrikes came in response to requests and information provided by Iraqi security forces on the ground near Fallujah, which is in IS control, and were done in coordination with Iraqi forces.

A senior U.S. defense official said there was fog in that area and that weather may have played a role in the incident. The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

Carter said he told Abadi that the U.S. was investigating and would work with the Iraqis.

Asked if he was worried the deaths might further anger Iraqi citizens who may not be happy with the American and coalition presence in Iraq, Carter said, "I hope Iraqis will understand that this is a reflection of things that happen in combat. But it's also a reflection of how closely we are working with the government" of Iraq.

He added that during the call, both he and Abadi recognized that "things like this can happen in war."

The U.S. military statement Friday said the strike may have resulted in the death of Iraqi soldiers. The statement did not say how many Iraqi soldiers may have been killed; other officials said the Iraqis initially reported that about 10 may have died.

When asked how many Iraqis may have been killed, the U.S. military command known as Combined Joint Task Force-Operation Inherent Resolve said, "The coalition is investigating the incident and will make further details available when appropriate."

Carter met with Abadi during a stop in Baghdad on Monday to discuss the fight against IS, and the U.S. and coalition plans to accelerate the campaign.