US President Donald Trump was on Tuesday set to announce whether he will scrap the 2015 nuclear deal with Iran.
France, Germany and Britain have launched a diplomatic offensive in recent weeks to try to persuade Trump not to scuttle the deal. But French President Emmanuel Macron has already said their efforts may have failed.
Trump was to make the announcement at the White House at 2 pm (1800 GMT).
Sealed by Trump's predecessor, president Barack Obama - with the support of China, Russia, Germany, France and Britain - the Joint Comprehensive Plan of Action (JPCOA) eased sanctions on Tehran in exchange for commitments to halt what Western powers feared was a nuclear weapons programme.
Trump said he opposed the agreement during the 2016 presidential campaign and in office has repeatedly threatened to pull the US out of the pact, last week calling it a "horrible deal."
British Foreign Secretary Boris Johnson said Monday that there was currently no viable alternative to the Iran deal.
"Plan B does not seem, to me, to be particularly well developed at this stage," he told broadcaster Fox News, said to be one of Trump's favourite channels, prior to a meeting with US Vice President Mike Pence.
If the deal was scrapped, Johnson said, "you have to answer the question, 'What next?' What if the Iranians do rush for a nuclear weapon?"
The French and German foreign ministers pleaded with the US to uphold the agreement after a meeting in Berlin on Monday.
There were "no alternatives" to the nuclear restrictions that Tehran had accepted, German Foreign Minister Heiko Maas said.
