Prime Minister Boris Johnson has told Iranian President Hassan Rouhani there should be “an end to hostilities”.
In a phone call, Mr Johnson urged calm following Iran’s retaliation for the US killing of army chief Qasem Soleimani.
The PM also underlined the UK’s commitment to the Iran nuclear deal, despite pressure to abandon it from US President Donald Trump.
It came after Iran fired missiles at air bases in Iraq where US and UK troops are stationed.
Mr Johnson also raised the “continued detention and mistreatment” of charity worker Nazanin Zaghari-Ratcliffe and other dual nationals who are imprisoned in Iran, and called on President Rouhani to organise their immediate release.
Downing Street said the phone call lasted around 20 minutes.
Earlier, UK Foreign Secretary Dominic Raab said he recognised the threat Iran poses in the Middle East and a US “right to self-defence”, after talks with his counterpart in Washington.
Mr Raab also reiterated his support for a diplomatic resolution to the crisis during his meeting with US Secretary of State Mike Pompeo.
“We want to see the tensions de-escalated,” he said.
The already tense relationship between the US and Iran has deteriorated significantly in recent days, after a US drone strike killed General Qasem Soleimani, one of Iran’s top military commanders.
Mr Raab welcomed US President Donald Trump’s call for a diplomatic resolution following Iran’s retaliatory missile strikes.
“Of course it also needs the government in Iran to be willing and committed to that outcome as well,” he added.
The US government said Gen Soleimani had been plotting “imminent attacks”, but Mr Raab refused to say whether he had seen any intelligence on this.
Mr Raab reiterated the UK’s commitment to the Joint Comprehensive Plan of Action (JCPOA) on restricting Iran’s nuclear programme,
The foreign secretary said the UK government was “looking very hard at what should happen next” after Iran declared earlier this week that it would abandon all limits on its enrichment of uranium.
“We are absolutely committed, as our American and European partners are, to avoiding Iran acquiring a nuclear weapon,” he said.
Iran’s announcement marked further fracturing of the 2015 nuclear deal, in which the country agreed to limit sensitive nuclear activities in return for the lifting of crippling economic sanctions.
The US withdrew from the deal in 2018, but the other parties – the UK, France, Germany, China and Russia – said they were still dedicated to it.
Mr Raab said that, while the UK has been committed to the deal, “we’ve reached a point where non-compliance has been so acute in the most recent steps taken by Iran.”
Asked whether there was still a chance that the deal could survive if Iran started to uphold its commitments, he said: “There is an opportunity to build on this deal.
“But ultimately the objective is the most important thing which is to avoid the risk of Iran seeking – let alone acquiring – a nuclear weapon.”