An Iran-backed militia vowed on Monday to retaliate for a series of American airstrikes against their fighters in Iraq and eastern Syria on Sunday, an attack that left at least 25 dead.

The US strikes came in response to a barrage of at least 30 rockets on Friday that struck a base used by American personnel in northern Iraq, killing one US military contractor, and injuring four.

The weekend violence raised fears of escalating armed conflict between the US and Iran in Iraq, which is already struggling to recover from the war against Isis and soldiering its way through an unprecedented political crisis, with weekly protests demanding the ousting of the government.

Iranian officials raged against the US attack, reportedly launched by combat drones on five positions used by Kataib Hezbollah.

The Iraqi Shia militia is part of the country’s official security forces as a unit in the Popular Mobilisation Units (PMU), which came to prominence in the fight against Isis.

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Iran’s foreign ministry spokesperson Abbas Mousavi accused the US of supporting “terrorism” by hitting a group which had fought against Isis in the five-year conflict against the jihadi group.

Abu-Mahdi al-Mohandes, a top commander of the PMU, warned of revenge for the attack, which also left 51 injured.

“The blood of the martyrs and injured will not come to naught and the response against the US forces in Iraq will be harsh,” he was quoted as saying.

The US military says it was responding to Friday’s attack at an airbase in northern Iraq. No one claimed responsibility for that attack, but security experts have said only Iranian-backed militias hostile to the US have the means, motive and positions to carry out such an assault.

“We strongly deny any role in the attack on the American forces,” Iranian government spokesperson Ali Rabiei was quoted as saying Monday. “This claim without proof cannot justify the bombing and killing of people against international law.”

The US said the strikes followed “repeated” attacks by Kataib Hezbollah on Iraqi bases that host US forces to “degrade” the group’s ability to carry out future operations against American troops, according to a statement by Pentagon spokesperson Jonathan Hoffman.

“Iran and their Kataib Hezbollah proxy forces must cease their attacks on US and coalition forces, and respect Iraq’s sovereignty, to prevent additional defensive actions by US forces,” the statement said.

But while the American allies in the region eager for a confrontation with Iran praised the attack, Iraq’s prime minister and president warned that it was a violation of its sovereignty. The PMU have become increasingly powerful politically, and are linked to lawmakers in the country’s parliament. Though some of its three dozen units have strong organisational and historic ties to Iran, others do not.