US President Donald Trump met his national security team to decide whether to put the 2015 Iran nuclear deal at risk by re-imposing American sanctions.
European leaders queued to beg Trump to renew the sanctions relief that Washington agreed to in the accord, which Trump has denounced as one of the worst of all time.
US officials expect Trump to grudgingly sign the sanction waivers once again before they start to expire this weekend - even if he also imposes new sanctions on Tehran over non-nuclear issues.
But the angst in Berlin, Brussels, London and Paris underlined what is at stake: the mercurial US president is quite capable of sabotaging the landmark agreement.
America's allies see the accord between six world powers and Tehran as the best way of preventing Iran's quest for nuclear arms and a victory for multilateral diplomacy.
But Trump argues his predecessor Barack Obama gave away too much to Iran in sanctions relief, without forcing the Islamic republic to end its ballistic missile program and aggressive support for militant groups.
"The president still strongly believes this is one of the worst deals of all time," White House spokeswoman Sarah Sanders told reporters as Trump's meeting started.
"One of the single greatest flaws is its restrictions leave Iran free... to openly develop their nuclear program and rapidly achieve a nuclear weapons breakout capability.
Sanders did not say when an announcement would be made. State Department officials initially suggested it could come as early as Thursday, but later said it would not happen before Friday.
"The secretary has been at the White House almost every day. He was there yesterday for many, many hours," Assistant Secretary of State for Steve Goldstein told reporters.
"The decision meeting is this afternoon," he said.
Trump has already declared that he think the Iran nuclear deal is no longer in the United States' national interest.
By thus "decertifying" the arrangement, he opened a window for the US Congress to reimpose sanctions, but to date, it has not done so - leaving the issue of the waivers.
So far, Trump has continued to follow Obama's lead in regularly signing sanctions waivers so that US economic measures against Tehran do not "snap back."
The deadlines for a number of these waivers to be renewed will fall over the coming week, and Trump now is obliged to decide whether or not to maintain sanctions relief.
If he does allow the punitive measures to go back into effect, Iran will accuse the United States of breaking the deal, under which Tehran accepted restrictions on its nuclear program.
European capitals will also be dismayed, having pressed Washington to accept that the deal was an international agreement and that Iran has abided by its terms.
French President Emmanuel Macron called Trump on Thursday and stressed France's determination to see "the strict application of the deal and the importance of all the signatories to respect it."
The White House said Trump had "underscored that Iran must stop its destabilizing activity in the region."
In Brussels, the European Union and the foreign ministers of Britain, Germany and France presented a united front after talks with Iranian Foreign Minister Mohammad Javad Zarif.
"The deal is working, it is delivering on its main goal which means keeping the Iranian nuclear program in check," EU foreign policy chief Federica Mogherini said.
The agreement, she said, is "making the world safer and... preventing a potential nuclear arms race in the region".
UN inspectors have certified Iran's compliance with the deal nine times, most recently in November.
"It is important now that all parties respect this joint commitment and that our American allies respect it as well," French Foreign Minister Jean-Yves Le Drian said.
Iran has said that if the US walks away from the agreement, it is ready to give an "appropriate and heavy response."
Zarif took to Twitter after the Brussels meeting to warn that "Iran's continued compliance (is) conditioned on full compliance by the US."
The Iranian said there was a "strong consensus in Brussels today" that Iran was complying with the deal and that "any move that undermines (the agreement) is unacceptable."
One of the criticisms levelled at the nuclear deal is that it does nothing to address Iran's continuing ballistic missile program and meddling in conflicts such as Yemen and Syria.
Europeans say these issues should be kept separate from discussion of the deal, but in a nod to US concerns, Mogherini stressed they were raised with Zarif on Thursday.