An airstrike by the American-led coalition fighting the Islamic State group that apparently killed a number of Iraqi soldiers Saturday seemed to be the result of “a mistake that involved both sides,” U.S. Defense Secretary Ash Carter told reporters, according to the Associated Press. Carter called Iraqi Prime Minister Haider al-Abadi to express condolences over the casualties, which were reported to have happened near the central city of Fallujah.

“We will conduct a thorough investigation to determine the facts,” the U.S. military said in a statement cited by Al Jazeera. “To the best of our knowledge, there have been no previous incidents of friendly fire in Iraq involving the coalition during the course of Operation Inherent Resolve.”

While the U.S. military hasn’t said how many may have been killed, Iraqi sources put the number killed at about 10. Iraqi security forces on the ground near Fallujah, an area where the Islamic State group is known to be active, provided the information that led to the airstrikes, the U.S. military said.

“These kinds of things happen when you're fighting side by side as we are,” CBS News quoted Carter as saying. The defense secretary didn’t provide many more details about the strikes, which were coordinated with Iraqi forces. An unnamed U.S. military official told CBS News bad weather could have been a factor in the incident.

The extremist group formerly known as either ISIL or ISIS began to take control of most of the Anbar province surrounding Fallujah during the summer of last year, as Al Jazeera reported. Iraqi Defense Minister Khaled al-Obeidi indicated Saturday the militant group could be driven out of the provincial capital of Ramadi, about 31 miles west of Fallujah, this month, Al Jazeera said.

Hakim al-Zamili, the chairman of the security and defense committee in the Iraqi parliament, said action should be taken against the pilot who conducted the airstrike, NBC News reported. “This is a heinous crime, and we cannot just let it go without sending to trial those who were behind it,” al-Zamili said.