The US is to controversially initiate a process at the UN Security Council to reinstate international sanctions on Iran lifted under a 2015 nuclear deal.
Secretary of State Mike Pompeo will submit a complaint accusing Iran of significant non-compliance and trigger the sanctions “snapback” mechanism.
However, other world powers insist he has no legal right to do so.
The US itself stopped complying with the accord two years ago, when President Donald Trump abandoned it.
Once the complaint has been submitted, other countries on the Security Council will have 30 days to adopt a resolution to avert the snapback. But, as a permanent member, the US will be able to exercise its veto power.
The Trump administration’s move comes a week after the council rejected its bid to extend indefinitely an arms embargo on Iran that is due to expire in October.
How did we get here?
The nuclear deal saw the P5+1 group of powers – the US, China, France, Russia, the UK and Germany – give Iran sanctions relief in return for limits on its sensitive activities and international inspections to show it was not developing nuclear weapons.
The accord has been close to collapse since the US withdrew and reinstated economic sanctions in 2018 in an attempt to force Iran to negotiate a replacement that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles.
Iran has so far refused and retaliated by rolling back key commitments, including those on the production of enriched uranium, which can be used to make reactor fuel but also nuclear warheads.
The five powers still party to the deal have tried to keep it alive, although the UK, France and Germany triggered a formal dispute mechanism over the Iranian breaches in January that could ultimately lead to the snapback of UN sanctions.
What does the US want?
After its defeat at the Security Council last week, US permanent representative Kelly Craft declared that the Trump administration would “stop at nothing to extend the arms embargo” on Iran.
On Wednesday, the president announced that the US intended to “restore virtually all of the previously suspended United Nations sanctions on Iran”.
“It’s a snapback. Not uncommon,” he told reporters in Washington. “My administration will not allow this Iran nuclear situation to go on. They will never have a nuclear weapon.”
Mr Pompeo stressed that under Security Council resolution 2231, which endorsed the nuclear deal, the US had a legal right to trigger the snapback.
“It has a set of provisions, it has a set of rights and obligations, and we will be in full compliance with that, and we have every expectation that every country in the world will live up to its obligations, including every member of the P5,” he said.
In addition to maintaining the arms embargo, the snapped-back sanctions would force Iran to suspend all nuclear enrichment-related and reprocessing activities, and ban imports of anything that could contribute to those activities or the development of nuclear weapon delivery systems.
Sanctions on dozens of individuals and entities would also be reinstated.
How have other countries reacted?
Iranian Foreign Minister Mohammad Javad Zarif tweeted on Sunday: “US recourse to Dispute Resolution Mechanism in 2231 has NO LEG TO STAND ON.”
The five other remaining parties to the nuclear deal, all of which currently sit on the Security Council, are also opposed to the US plan.
Russian Deputy Foreign Minister Sergei Ryabkov said on Thursday that it was “absurd”, adding that the administration had no legal grounds to do so.
European countries have said that President Trump made it clear in 2018 that the US had ended its participation in the nuclear deal and given up any rights.
It is not clear how the other countries might try to stop the US.
All UN member states would be obligated to enforce the snapped-back sanctions, although diplomats told Reuters news agency that some might refuse to do so.