A US aircraft appears to have mistakenly carried out an air strike that killed Iraqi security forces near the city of Fallujah, US Secretary of Defense Ash Carter said.
Iraq's Minister of Defence Khaled al-Obeidi said earlier that nine soldiers died in the strike yesterday.
The US military is leading an international coalition that is waging an air offensive on the so-called Islamic State in Iraq and Syria.
Mr Obeidi told a news conference that the strike occurred when coalition air forces were covering the advance of Iraqi ground troops near Fallujah because Iraqi army helicopters were unable to fly due to bad weather.
Mr Carter said he spoke with Iraqi Prime Minister Haider al-Abadi by phone to express his condolences over the deaths. He said it appeared a US aircraft had carried out the air strike.
"That's the information I have now," he said.
"He (Mr Obeidi) 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."
Mr Carter met with Mr Obeidi this week during a brief visit to Baghdad to assess the state of the campaign against IS.
