Boris Johnson has spoken with Iran's president and called for an "end to hostilities" in the Gulf, Downing Street has said.
The prime minister had a 20-minute discussion with Hassan Rouhani by phone on Thursday morning, Number 10 said.
A spokesman said the pair "discussed the situation in the region" following the US killing of Iranian general Qassem Soleimani.
They added that Mr Johnson "underlined the UK's continued commitment" to the nuclear deal struck by the West and Iran - despite US President Donald Trump calling for the UK to follow his lead and abandon the agreement.
The prime minister also raised the "continued detention of mistreatment of Nazanin Zaghari-Ratcliffe and other dual nationals" - calling for their immediate release.
The assassination of Maj Gen Soleimani last week has ratcheted up tensions between Washington and Tehran.
Iran responded to the US airstrike near the Iraqi capital Baghdad by launching missile strikes against bases housing US and coalition troops in the country.
Mr Johnson has stood behind Mr Trump's targeting of the Iranian general, telling the Commons on Wednesday that Maj Gen Soleimani had the "blood of British troops on his hands".
Mr Trump said in the wake of Iran's response that Tehran appeared to be "standing down".
But a senior Iranian commander, quoted by the Tasnim news agency, has said they will take "harsher revenge soon" for Maj Gen Soleimani's death.
Foreign Secretary Dominic Raab held talks with US Secretary of State Mike Pompeo on Wednesday.
He said the UK "recognises the US right to self-defence and the serious threat Iran poses in the region".
Mr Raab added: "We need to de-escalate tensions, and we discussed the need to find a diplomatic way forward."