No 10 has urged Iraq to allow UK troops to stay in the country following the US assassination of Iranian general Qasem Soleimani, saying their work is vital.
Soleimani was killed in a US drone strike in Iraq on Friday on the orders of President Donald Trump.
Iraqi MPs responded to the strike by passing a non-binding resolution calling for an end to the foreign military presence in their country.
European leaders have called for all sides to show restraint.
Boris Johnson will chair a meeting of senior ministers later to discuss the deepening crisis and spoke to Iraqi Prime Minister Adel Abdul Mahdi by phone on Monday morning.
A Downing Street spokesperson said the leaders discussed the need to “deescalate tensions in the region” and “agreed to work together to find a diplomatic way forward”.
“The prime minister underlined the UK’s unwavering commitment to Iraq’s stability and sovereignty and emphasised the importance of the continued fight against the shared threat from Daesh [the Islamic State group]”.
About 400 British troops are stationed in Iraq, while the US has 5,200.
Caretaker Iraqi Prime Minister Mr Abdul Mahdi spoke in favour of US and other foreign forces leaving the country, although most Sunni and Kurdish MPs boycotted the vote.
A UK government spokesman said that coalition forces were in Iraq to protect its people and others from the Islamic State group, at the request of the Iraqi government.
Meanwhile, a British frigate and destroyer – 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.
In a joint statement issued on Sunday night, Mr Johnson, Germany’s Angela Merkel and France’s Emmanuel Macron said the current cycle of violence “must be stopped” 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, Iran has responded by vowing revenge and announcing it will no longer abide by the restrictions in its 2015 nuclear deal.
The deal limited Iranian nuclear capacities in exchange for the lifting of economic sanctions.
A No 10 spokesman said Iran’s announcement was “extremely concerning”.
“It’s in everyone’s interest that the deal remains in place,” he said, adding that “it makes the world safer”.
“We’ve always said the nuclear deal is a reciprocal deal and in light of Iran’s announcement we are urgently speaking to partners about next steps,” the spokesman said.
In their statement, the three European leaders urged the country to “reverse all measures inconsistent with” the deal.
Mr Johnson said he spoke to President 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 “reaffirmed the close alliance between the two countries”.
Following warnings from Iran, Mr Trump said that the US would respond in the event of retaliation for Soleimani’s death, “perhaps in a disproportionate manner”.
He repeated a threat to target Iranian cultural sites, saying the US would “strike very fast and very hard” if Tehran attacked Americans or US assets.
In response, a No 10 spokesman said: “There are international conventions in place that prevent the destruction of cultural heritage.”
Meanwhile, Iran’s ambassador to the UK has strongly denied reports in the Times that his country had threatened to kill British troops following the assassination of Soleimani.
The paper quotes an unnamed senior commander in Iran’s Revolutionary Guard as saying that its forces would “target US troops in the Middle East without any concern about killing its allies, including UK troops”.
But Hamid Baeidinejad described the story as “provocative” and a “vicious lie” in a Twitter post.
“I will ask the concerned UK authorities to take swift action to stop such malicious false propaganda in this very sensitive time,” he said.
An Iranian Embassy official also told the BBC there were no plans to attack British targets and said any suggestion that an attack could take place in Britain was laughable, adding “we are not idiots”.
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.
On Sunday Mr Johnson said Soleimani 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,” he 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.
On Sunday Foreign Secretary Dominic Raab defended the assassination 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 rebutted criticism of Mr Johnson for being on holiday as the crisis unfolded, telling the BBC 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.