Boris Johnson has joined European leaders in a call for all sides to show restraint after the US assassination of Iranian general Qasem Soleimani.
In a joint statement with Germany’s Angela Merkel and France’s Emmanuel Macron they said the current cycle of violence “must be stopped”.
Mr Johnson will meet key ministers later and has discussed the deepening crisis with US President Donald Trump.
Iran has vowed revenge after General Soleimani was killed in a US strike.
In the joint statement, issued on Sunday night, Mr Johnson and his French and German counterparts urged restraint and called on Iran to refrain from further violent action and proliferation.
The three leaders said they were concerned by the “negative” role Iran has played in the region but called on “all parties to exercise utmost restraint and responsibility”.
With tensions rising in the region following the drone strike ordered by President Trump, Iran has responded by vowing revenge and announcing it will no longer abide by the restrictions in its 2015 nuclear deal.
In the statement, the three leaders urged the country to “reverse all measures inconsistent with” the deal.
Meanwhile, Mr Johnson is preparing to assemble key ministers to discuss the spiralling crisis in the Middle East.
The prime minister said he spoke to Mr Trump on Sunday about the assassination of the Iranian general, who spearheaded the country’s military operations in the Middle East as head of the elite Quds Force.
The White House said the two leaders had discussed the current situation and “reaffirmed the close alliance between the two countries”.
Earlier on Sunday, in his first public statement since Soleimani’s death, Mr Johnson said the 62-year-old had been “responsible for a pattern of disruptive, destabilising behaviour in the region”.
“Given the leading role he has played in actions that have led to the deaths of thousands of innocent civilians and western personnel, we will not lament his death,” Mr Johnson said.
“It is clear, however, that all calls for retaliation or reprisals will simply lead to more violence in the region and they are in no-one’s interest.”
Mr Johnson said the UK was in “close contact” with all sides to encourage de-escalation and said Parliament will be updated when it returns on Tuesday.
On Monday, former foreign secretary Jeremy Hunt told BBC Breakfast one “heartbreaking” result of the crisis was that it is now going to be “much harder” to secure the release of British-Iranian woman Nazanin Zaghari-Ratcliffe.
She is jailed in Iran over spying allegations that she denies.
Mr Hunt also said the UK and Europe needs to recognise there is a “fault line” in the Western alliance due to a “disparity in defence spending”.
“When it comes to big matters of global security we’re not putting our money where our mouth is,” he said.
Mr Trump has made repeated complaints over the defence spending of other Nato allies.
In terms of its gross domestic product (GDP, the total value of goods produced and services) the US spent roughly 3.4% on defence in 2019, according to Nato estimates, while the average in European Nato countries and Canada was 1.55%.
Meanwhile, retired army officer Sir Simon Vincent Mayall warned on Radio 4’s Today programme that British troops serving in the Middle East could “possibly” be killed in retaliation attacks on US soldiers.
The Ministry of Defence adviser said western allied troops stationed in neighbouring Iraq were “joined at the hip” and that casualties could be shared in Iraq if Iran hits back.
Iraqi MPs have responded to the drone strike by passing a non-binding resolution calling for an end to the foreign military presence in their country.
Caretaker Prime Minister Adel Abdul Mahdi spoke in favour of US and other foreign forces leaving, although most Sunni and Kurdish MPs boycotted the vote.
About 400 British troops are stationed in Iraq, while the US has 5,200.
A UK government spokesman said that coalition forces were in Iraq to protect its people and others from the Islamic State group.
“We urge the Iraqi government to ensure the coalition is able to continue our vital work countering this shared threat,” he said.
Meanwhile, HMS Montrose and HMS Defender are to start accompanying UK-flagged ships through the Strait of Hormuz in the Gulf, where a tanker was seized by Iran last July.
Foreign Secretary Dominic Raab, who told the BBC’s Andrew Marr show that he learned of the US attack on Soleimani “as it happened”, spoke to the Iraqi prime minister on Sunday morning.
Mr Raab defended the killing because of the US’s “right to self-defence” against Soleimani’s use of militia’s to destabilise the region and attack Western forces.
He also defended Mr Johnson for being on holiday as the crisis unfolded, saying that he had been “in constant contact with the prime minister over the Christmas break on a whole range of foreign policy issues”.
Shadow foreign secretary and Labour leadership candidate Emily Thornberry accused the prime minister of “sunning himself” while the chief civil servant chaired three meetings of Cobra, the government’s emergency response committee.
Shadow justice secretary Richard Burgon, who is standing to be Labour deputy leader, said Mr Johnson’s response was “pathetic”, adding that he should stand up to a US president “recklessly threatening to launch a war”.