Disagreements over the weapons embargo on Iran loom large before a visit by US Secretary of State Mike Pompeo to Britain on Tuesday.
Mr Pompeo, who is leading a diplomatic offensive for an extension to the embargo on Tehran, will arrive in London to meet British officials before heading to Copenhagen on Wednesday.
The US State Department said he would meet Prime Minister Boris Johnson and UK Foreign Secretary Dominic Raab to discuss global priorities.
The US is looking at a probable defeat in the UN Security Council and disagreements with Russia and China over the 13-year-old arms restrictions.
They are due to expire in October under the terms of Tehran’s 2015 nuclear deal with world powers.
In 2018, the US withdrew from the deal that lifted economic sanctions on Iran in return for a promise to relinquish its nuclear weapons programme.
The end of the arms embargo is one of the few face-saving benefits in the agreement to which Iran has been able to cling since the US withdrawal.
The US has threatened to return all UN sanctions to Iran if the Security Council does not grant an extension.
The extension of the embargo would kill the 2015 deal and Britain and its European allies have vowed to try to maintain it, despite pressure from the US.
In June, Britain, France and Germany committed to upholding the nuclear non-proliferation regime, “ensuring that Iran never develops a nuclear weapon”.
They said the nuclear deal “plays a key role in this respect”.
The US has insisted that lifting the embargo presents a clear threat.
“Given the Iranian regime’s history of resorting to terrorism and violence, perhaps we should take this threat seriously,” Mr Pompeo told the Security Council at the end of June.
“Renewing the embargo will exert more pressure on Tehran to start behaving like a normal nation."
