An air strike by the US-led coalition during an offensive against the so-called Islamic State killed nine Iraqi soldiers, including an officer, Iraq's defence ministry has confirmed.
Coalition strikes were called in to support an advance by ground troops near Fallujah on Friday but failed to note the scale of their advance.
US Defence Secretary Ash Carter called Iraq's PM to express his condolences.
Iraqi forces have been battling militants in Anbar province.
Iraqi Defence Minister Khaled al-Obeidi said: "The coalition air forces were covering the advance of army ground troops near Fallujah because the Iraqi army helicopters were not able to fly due to the bad weather."
Coalition air forces had carried out two strikes, enabling Iraqi soldiers to advance rapidly and engage jihadists in close combat.
"A third coalition strike carried out without taking into account the distance that had been covered resulted in casualties among our forces too," Iraq's joint operations command said in a statement.
Mr Carter told reporters on Saturday that the air strike appeared to be a "mistake that involved both sides", the Associated Press news agency reported.
The US military said it happened "despite co-ordination with Iraqi security forces on the ground".
Mr Carter called Iraq's Prime Minister Haider al-Abadi following the incident.
A US military statement earlier said: "The coalition offers condolences on the unfortunate loss of life of brave Iraq security forces on the front lines in the fight against Daesh [an alternative name to Islamic State]."
The US military said it was the first reported incident of so-called "friendly fire" during Operation Inherent Resolve, which aims to drive IS militants out of Anbar province.