"We condemn this attack on Iraqi military bases hosting Coalition -- including British -- forces," said Foreign Secretary Dominic Raab.

"We are concerned by reports of casualties and use of ballistic missiles."

It was unclear what casualties Raab was referring to. But a separate Ministry of Defence statement said: "We can confirm that there were no UK casualties."

Officials were "urgently working to establish the facts on the ground", it added.

The Pentagon meanwhile said it was still "working on initial battle damage assessments", with no immediate reports on casualties.

The Iraqi military said it sustained no casualties in 22 missile strikes on bases housing US troops.

Iran fired more than a dozen ballistic missiles overnight Tuesday-Wednesday at Iraqi bases housing the US and British military, officials in Washington and Tehran said.

It was the first act of the Islamic republic's promised revenge for the US killing of top Iranian general Qasem Soleimani last week.

Launched from Iranian territory and by Iranian forces, not a proxy, the attack marked a new turn in the intensifying confrontation between the United States and Iran.

Raab urged Tehran to show restraint. "We urge Iran not to repeat these reckless and dangerous attacks, and instead to pursue urgent de-escalation," he said.

"A war in the Middle East would only benefit Daesh and other terrorist groups," he added, referring to the Islamic State group.

Britain has about 1,400 UK military and civilian personnel based in Iraq as part of the 67-nation coalition fighting IS, according to the MoD.

The 400-strong troop contingent from two regiments are not involved in combat operations and instead provide training and equipment to Iraqi and Kurdish security forces.