WASHINGTON • An air strike that mistakenly killed Iraqi troops last Friday was carried out by an American plane, US officials said.
The episode poses a fresh challenge to the Obama administration's efforts to expand cooperation with the Iraqis in fighting the Islamic State in Iraq and Syria (ISIS), and Defence Secretary Ash Carter was quick to call Iraqi Prime Minister Haider al-Abadi to express his condolences and invite Iraq to participate in an investigation.
Mr Carter did not offer a detailed explanation for the errant air strike, which killed nine Iraqi soldiers and one officer, according to Iraq's Defence Minister. But Mr Carter said "it seemed to be a mistake that involved both sides".
Another US official, who asked for anonymity, said the plane that carried out the attack was a B-1B bomber, which dropped several bombs as it was supporting Iraqi forces battling ISIS fighters near Fallujah. At least one of those bombs, it appears, struck the Iraqi troops.
Bad weather may have been a contributing factor too, US and Iraqi officials say, making it difficult for reconnaissance drones or aircraft to keep track of shifting Iraqi positions.
A third US official said the Iraqi troops appeared to have been closer to the target area than the Americans had realised.
There have been few instances of "friendly fire" in Iraq since the US-led campaign against ISIS began last year.
One key question raised by the attack is whether the risk of such episodes might be reduced by deploying teams of air controllers with the Iraqi forces.
To minimise the risk of US casualties, the Obama administration decided that the US advisers and trainers who are assisting Iraqi troops would work inside the confines of bases. But that has meant that US commanders have had to rely on aircraft to confirm the targeting information provided by Iraqi troops, instead of on US air controllers on the battlefield.
A broader political question is whether the episode will make it harder for Mr al-Abadi to accept an expanded role for the US, which wants to speed up the campaign against ISIS.
Mr Al-Abadi has come under pressure from Shi'ite politicians close to Iran to reject a greater US military role in Iraq. He did not take up the Pentagon on its offer to support Iraqi troops with Apache attack helicopter to reclaim Ramadi when he met with Mr Carter last Wednesday.
The political dynamics in Iraq were clear with the divergent response to the air strike. Mr Hakim al-Zamili, chairman of the Iraqi Parliament's defence and security committee and a political supporter of anti-American Shi'ite cleric Moktada al-Sadr, gave a higher toll than the Defence Ministry's and demanded an investigation.
Mr Al-Abadi, however, accepted Mr Carter's explanation that the episode was a genuine mistake, and expressed hope that politicians in Iraq and elsewhere would not try to exploit the air strike for their own purposes, according to the Americans' account of the phone call from Mr Carter to Mr al-Abadi.
There are signs, however, that the political debate over the incident might linger, as even Iraqi officials who work closely with the Americans say there must be strict accountability.
NEW YORK TIMES