Donald Trump has spoken with Boris Johnson to discuss the situation in Iraq and Iran, the White House has said in a statement.
The release offered few details of the specifics of the call on Sunday, noting only that the two leaders "reaffirmed the close alliance between the two countries."
Mr Johnson is expected to brief ministers today on the crisis in the Middle East.
On Sunday, Mr Trump insisted that Iranian cultural sites would be fair game as military targets if Iran carries through on its vow to attack Americans.
And he warned Iraq that the US would levy punishing sanctions if it expelled American troops in retaliation for a US strike in Baghdad that killed top Iranian official Major General Qassem Soleimani.
The US leader first suggested the idea of targeting Iranian cultural sites in a tweet on Saturday.
An Iranian government minister denounced Mr Trump as a "terrorist in a suit" after the US president sent a series of tweets threatening to hit 52 Iranian sites.
Speaking to reporters aboard Air Force One on route to Washington from Florida on Sunday, Mr Trump stood by his comments.
He said: "They're allowed to kill our people. They're allowed to torture and maim our people. They're allowed to use roadside bombs and blow up our people. And we're not allowed to touch their cultural sites? It doesn't work that way."
Meanwhile, Mr Johnson and his French and German counterparts, Emmanuel Macron and Angela Merkel, have called for all sides to work towards an urgent easing of tensions in the Persian Gulf.
In a statement, the three leaders said they were concerned by the "negative" role Iran has played in the region, including through forces by Soleimani, whose killing by the US sparked the crisis.
They said there was now "an urgent need for de-escalation".
"We call on all parties to exercise utmost restraint and responsibility. The current cycle of violence in Iraq must be stopped," the joint statement said.
"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)."
It added: "We stand ready to continue our engagement with all sides in order to contribute to defuse tensions and restore stability to the region."
The targeted killing of Soleimani sparked outrage in the Middle East, including Iraq, where more than 5,000 troops are still on the ground 17 years after the US invasion.
Iraq is intending to expel foreign troops following Friday's deadly airstrike in Baghdad.
The Iraqi parliament also wants the government to ensure foreign troops do not use its land, air and waters for any reason.
It came after Iraq's foreign ministry denounced the US drone attack as a "blatant" violation of sovereignty and a breach of the agreement between Iraq and the US-led coalition.
The US has said it is "disappointed" by the vote in Iraq's parliament and urged the country's leaders to consider the importance of the US-Iraq economic and security relationship, as well as the role of the US-led coalition in defeating Islamic State.
Mr Trump said the US would not leave without being paid for its military investments in Iraq over the years - adding that if the troops do have to withdraw, he would levy punishing economic penalties on Baghdad.
"We will charge them sanctions like they've never seen before ever. It'll make Iranian sanctions look somewhat tame," he said.
"If there's any hostility, that they do anything we think is inappropriate, we are going to put sanctions on Iraq, very big sanctions on Iraq.
"We're not leaving until they pay us back for it."
Iranian state television reported that Iran will no longer abide by any limits of the 2015 nuclear deal it signed with the US and other world powers.
Mr Trump withdrew the US from the deal in 2018 and stepped up economic sanctions on Tehran, fuelling hostilities that lead to the Soleimani killing.
In other developments:
- Two Katyusha rockets have fallen inside Baghdad's green zone, Iraq's military has said
- The military also confirmed a third rocket fell in the nearby Jadriya area
- Hundreds of thousands of mourners have flooded the streets of Iran to walk alongside a coffin carrying the remains of Qassem Soleimani
- Tehran has announced new steps to distance itself from a 2015 nuclear deal with major powers - but says this can be reversed if the US lifts sanctions
- The US-led coalition in Iraq is pausing operations in support of Iraqi forces in the fight against IS militants
- A former leader of Iran's Revolutionary Guards has said the Israeli cities of Tel Aviv and Haifa could be targeted to avenge the death of Maj Gen Soleimani
- Iraqi President Barham Salih offered his Iranian counterpart his condolences over the death of the general in a phone call
- NATO will hold an urgent meeting at ambassador level on the Iraq-Iran crisis on Monday
- General David Petraeus, a former director of the CIA, says the killing of Iran's top general is such a significant escalation that it may prompt Tehran to "give pause"
- Breaking his silence after returning from holiday in the Caribbean, Boris Johnson has said the UK "will not lament" Maj Gen Soleimani's death
Republicans in Congress have generally backed Mr Trump's move to authorise the killing of Soleimani, who has long been seen as a threat by the US authorities.
US Secretary of State Mike Pompeo defended targeting Soleimani in Friday morning's airstrike outside Baghdad airport, telling ABC News the intelligence assessment on Iran's effective second-in-command was "clear".
He added that the US will respond with "lawful strikes" against any retaliatory attacks on American targets.