A drone packed with explosives hit an Iraqi base housing US troops early on Saturday, causing damage but no casualties, the Iraqi military and the US-led coalition said.
The attack on the Ain Al Asad air base was the fourth targeting US troops in Iraq in less than a week, as an armed campaign blamed on pro-Iranian groups intensifies.
"Each attack... undermines the authority of Iraqi institutions, the rule of law and Iraqi national sovereignty," said coalition spokesman Col Wayne Marotto.
"No injuries reported. A hangar was damaged," he said in a tweet.
Coalition officials often says Iraq's sovereignty has been "violated" following attacks by Iran-backed militias.
It was only the second time authorities publicly confirmed a drone had been used in an attack on a target inside Iraq.
In April, a drone packed with explosives hit the coalition's Iraq headquarters in the military part of the airport in the Kurdish regional capital Erbil.
That attack sent shock waves around Iraq – the tactic poses a headache for the coalition, as drones can evade the C-RAM air defences it has installed to protect its bases.
But an Iraqi government official told AFP that the Erbil strike was not the first use of a drone against a target inside Iraq.
"Drones were used by pro-Iran groups more than once before the attack in Erbil," the official said, speaking on condition of anonymity.
In the meantime, there has been no let-up in rocket and other attacks carried out by the coalition's foes over the past 18 months.
Since last Sunday, Iran-backed militias have fired two rockets at Ain Al Assad, six at the Balad air base and two at Baghdad airport, all of which host coalition troops.
The US embassy in Baghdad and coalition supply convoys have also come under repeated attack.
Many of the attacks have been carried out in the name of shadowy groups regarded as little more than cover names for the main pro-Iran factions. Others have gone unclaimed.