The UK and US have warned Iran it will face further sanctions if it continues to move back towards a nuclear weapons programme.
The US Secretary of State Mike Pompeo arrived in London for talks with the Prime Minister and with Foreign Secretary Jeremy Hunt - just hours after Iran announced it could resume higher uranium enrichment within weeks.
Iran's President Hassan Rouhani wrote to the UK, France, Germany and other signatories of the 2015 deal, to say that following America’s withdrawal exactly a year ago, he too would stop complying with key parts of the deal - unless new terms are agreed in 60 days.
Mr Hunt said:
"For as long as Iran keeps its commitments, then so too will the United Kingdom."
Iran was not expected to be the focus of the visit.
The US is unhappy at reports Britain could let Chinese tech giant Huawei bid to supply part of its 5G network.
It has already threatened to limit intelligence sharing if it does win contracts.
Mr Pompeo insists that, whatever happens, no action the UK takes on either Huawei or Iran will threaten the special relationship between the US and UK.
Citing the history of Winston Churchill "staring evil in the face" in the Second World War, the Secretary of State said:
"We're on the same side, we're on the side of values-driven democracy, we're on the side of freedom."
On Monday, Washington deployed an aircraft carrier and B-52 bombers to the Middle East, in what the US national security advisor said was a warning to Iran.
The move followed "clear indications" that Iranian and Iranian proxy forces were preparing to possibly attack US forces in the region, a defence official told the Associated Press.
US forces at sea and on land were thought to be the potential targets.