Taking questions in a weekly House of Commons session on Wednesday, Johnson threw his support behind Washington as US-Iran tensions soar.
Early on Wednesday, Iran attacked military bases in Iraq housing US troops in retaliation for the recent killing of Soleimani near Baghdad International Airport, which had been ordered by US President Donald Trump.
Johnson said no British personnel were injured in the Iranian attacks, and that the UK was doing “everything we can” to protect its interests in the region.
Iran’s missile attacks on military bases in Iraq were “reckless and dangerous”, Johnson said, as he called for “urgent de-escalation”.
“Iran should not repeat these reckless and dangerous attacks but should instead pursue urgent de-escalation,” he told Parliament.
Johnson, who had been criticised for not cutting short his holiday in the Caribbean to address the escalating crisis, said the UK was working hard to “dial this thing down”.
Johnson alleged Soleimani was involved in arming Yemen’s Iran-aligned Houthi movement and Hezbollah.
He also said Soleimani supported the government of Syrian President Bashar al-Assad and was behind attacks on British troops.
Labour Party leader Jeremy Corbyn asked Johnson about the legality of “killing somebody in a foreign territory”, and said the US’s assassination of Soleimani “should be condemned as such” by the British government.
Johnson replied: “[The] strict issue of legality is not for the UK to determine, as it wasn’t our operation.
“Most reasonable people would accept the US has a right to protect its bases and personnel.”
Early on Wednesday, Iran’s Islamic Revolutionary Guard Corps (IRGC) launched more than a dozen ballistic missiles targeting at least two Iraqi military bases where US military and coalition personnel were stationed.
The UK has about 1,400 UK military and civilian personnel in Iraq as part of the 67-nation coalition fighting the ISIL (ISIS) group, according to the UK 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 out of Baghdad, 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.