Boris Johnson and his French and German counterparts have called for all sides to work towards an urgent easing of tensions in the Persian Gulf.
The Prime Minister, Emmanuel Macron and Angela Merkel said that while they were concerned by the “negative” role Iran has played in the region – including through forces directed by Qassem Soleimani – there was now “an urgent need for de-escalation”.
Gen Soleimani’s assassination by the United States last Friday sparked the crisis, and prompted retaliation threats from the Iranian regime.
In a joint statement released late on Sunday, the three leaders said: “We call on all parties to exercise utmost restraint and responsibility. The current cycle of violence in Iraq must be stopped.
“We specifically call on Iran to refrain from further violent action or proliferation, and urge Iran to reverse all measures inconsistent with the JCPOA (the Iran nuclear deal).
“We recall our attachment to the sovereignty and security of Iraq. Another crisis risks jeopardising years of efforts to stabilise Iraq.
“We also reaffirm our commitment to continue the fight against Daesh (the so-called Islamic State), which remains a high priority. The preservation of the Coalition is key in this regard. We therefore urge the Iraqi authorities to continue providing the Coalition all the necessary support.
“We stand ready to continue our engagement with all sides in order to contribute to defuse tensions and restore stability to the region.”
Around 400 UK troops are stationed in Iraq in the fight against IS, while the US has 5,200, prompting fears of a withdrawal that could cripple the battle against the terror group.
Mr Johnson is to assemble key ministers to discuss the spiralling crisis on Monday afternoon, and is also likely to continue diplomatic discussions with world leaders.
Middle East Minister Andrew Murrison called on players involved to “cool it”, telling Sky News: “I think the Americans are trying to make sure this doesn’t escalate in the sense of Iran taking measures which are disproportionate and which may cause, inevitably, this thing to go on and on and get worse.
“The problem with this is there is a risk of miscalculation and reaching a point which is very difficult to reverse.
“So I would urge all concerned to cool it and that has been the consistent refrain of the UK Government trying to dial down the temperature on this and urge de-escalation.”
Elsewhere, a former US ambassador to Iraq, Afghanistan and Syria criticised the decision not to inform Britain about the strike.
Ryan Crocker told BBC Radio 4’s Today programme he was “concerned” about the White House’s failure to warn allies before it took action.
“Britain is and has been for a number of years our foremost ally,” he said. “When we take an action like this it can have great consequences for our allies. We, I think, owe it to them to consult in advance.”
After speaking to the US President on Sunday, Mr Johnson broke his silence to say the UK “will not lament” the death of the leader who he said was “a threat to all our interests”.
Urging de-escalation from all sides, Mr Johnson said calls for reprisals “will simply lead to more violence in the region and they are in no one’s interest” after the killing in Baghdad on Friday.
But a short while later, Mr Trump threatened to retaliate “perhaps in a disproportionate manner” if Iran strikes a US citizen or target.
Meanwhile, Iran announced it will abandon the limits in the unravelling nuclear deal on fuel enrichment, its uranium stockpile and research activities in a move that could bring it closer to assembling an atomic bomb.
And The Times quoted an unnamed senior commander in the elite Quds Force, which Gen Soleimani commanded, as warning British soldiers could be fatally attacked as collateral.
“Our forces will retaliate and target US troops in (the) Middle East without any concern about killing its allies, including UK troops, as this has turned in to a fully-fledged war with much collateral damage expected,” the commander said.
The PM said he will be speaking to Iraq “to support peace and stability” after its parliament called for the expulsion of foreign troops, including British soldiers working against IS.
Mr Johnson spoke to President Macron and Chancellor Merkel after arriving back in the UK on Sunday morning from his Caribbean holiday amid mounting criticism.
Meanwhile, Iraq’s parliament passed a non-legally binding bill calling for the expulsion of all foreign forces.
The Ministry of Defence was understood to be awaiting the decision of the Iraqi government before acting over UK soldiers based there as part of the US-led coalition.
A UK Government spokesman said: “We urge the Iraqi government to ensure the coalition is able to continue our vital work countering this shared threat.”