Iran fired more than a dozen ballistic missiles early Wednesday in retaliation for the US missile strike that killed its top general Qasem Soleimani last Friday.

Tehran has warned it will hit back harder if Washington responds.

But Johnson told parliament: "Iran should not repeat these reckless and dangerous attacks but should instead pursue urgent de-escalation."

Foreign Secretary Dominic Raab earlier said the Iraqi military bases hosting coalition forces included British troops. But there were no UK casualties.

Johnson, who has been criticised for not cutting short his holiday in the Caribbean to address the escalating crisis, said Britain was working hard to "dial this thing down".

At his weekly question and answer session, he defended the US strike, and promised to support the security of people in Iraq.

He alleged that Soleimani had armed Huthi rebels in Yemen and Lebanese militant group Hezbollah, and supported Syria's Bashar al-Assad.

Soleimani also supplied improvised explosive devices "to terrorists to kill and maim" UK forces, Johnson told MPs.

"That man had the blood of British troops on his hands," he said.

But he rejected suggestions from the main opposition Labour leader Jeremy Corbyn that Soleimani's killing was illegal under international law.

"The issue of legality is not for the UK to determine as it was not our operation," he said.

"But most reasonable people would accept that the United States has a right to protect its bases and personnel."

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 Ministry of Defence.

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.

"Non-essential" staff have been relocated while two Royal Navy warships are in the area on an "enhanced state of readiness" to protect UK ships in the Strait of Hormuz, said Johnson.