The U.S. military carried out air strikes on Sunday against the Kataib Hezbollah militia group in response to the killing of a U.S. civilian contractor in a rocket attack on an Iraqi military base, officials said.

Iraqi security and militia sources said at least 25 militia fighters were killed and at least 55 wounded following three U.S. air strikes in Iraq.

“The blood of the martyrs will not be in vain and our response will be very tough on the American forces in Iraq,” senior commander Jamal Jaafar Ibrahimi, known by his nom de guerre Abu Mahdi al-Mohandes, said late on Sunday.

Mohandes is a senior commander of Iraq’s Popular Mobilisation Forces (PMF), an umbrella grouping of paramilitary groups mostly consisting of the Shi’ite militias that was formally integrated into Iraq’s armed forces.

He is also one of Iran’s most powerful allies in Iraq and formerly headed Kataib Hezbollah, which he founded.

Iraqi security sources said on Monday that U.S. forces in Iraq’s northerly Nineveh province were ramping up security overnight, with U.S.-led coalition jets circling the perimeter of its military bases in Mosul and Qayarah.

“We strongly deny any role in the attack on American forces. This claim without any evidence cannot justify bombing and killing people in violation of international law,” said Iranian government spokesman Ali Rabiei, quoted by the semi-official news agency Fars.

Iran’s foreign ministry called on the United States to respect Iraq’s sovereignty and territorial integrity.

The air strikes come at a troubled time of protests in Iraq with thousands taking to the streets to condemn, among other things, militias such as Kataib Hezbollah and their Iranian backers. They also demand an overhaul of a political system they see as corrupt and keeping most Iraqis in poverty. More than 450 people have been killed in unrest as security forces have sought to quell anti-government demonstrations.

The PMF bolstered Iraq’s security forces during their battle to retake a third of the country from Islamic State, helping secure victory against the militants.

They were later formally integrated into Iraq’s official security structure and also wield large political influence.

Reporter’s code: 50101