The airstrike killed nine Iraqi service members, the Iraqi Defense Ministry confirmed Saturday.

In a statement, the U.S. Central Command described the incident Saturday as the first “friendly fire in Iraq involving the Coalition during the course of Operation Inherent Resolve”, referring to the U.S.-led fight against Daesh in Iraq and Syria.

“Despite coordination with the Iraqi security forces on the ground, initial reports indicate the possibility one of the strikes resulted in the death of Iraqi soldiers,” the statement by CENTCOM said.

It said the airstrike was conducted at the request of Iraqi security forces on the ground near Fallujah.

U.S. media outlets reported U.S. Secretary of Defense Ash Carter as calling the incident “a mistake that involved both sides”.

“Prime Minister [of Iraq Haider al-] Abadi and I agreed that this was an event that we both regretted and that there would be an investigation of it, but that these kinds of things happen when you’re fighting side by side,” Carter was quoted as saying.

Pentagon spokesman Peter Cook said in a statement that the secretary phoned Prime Minister Abadi on Saturday to pay condolences on the death of Iraqi security forces noting that both leaders agreed on a joint investigation into the case.

“The secretary and the prime minister committed to work together to prevent such incidents in the future and to ensure that this incident is not exploited by those who wish to create divisions between the United States and Iraq,” Cook said.

The CENTCOM statement noted that all airstrikes are conducted with the approval of the Iraqi government, adding that it has “formally invited Iraqi participation in the investigation”.