Western countries participating in the Iran nuclear deal have warned Tehran there will be serious consequences if it takes steps that violate its terms.
The UK, France and Germany said they would still back the deal as long as Iran upheld its commitments.
It follows Iran's move to suspend some commitments now the US has left the deal and begun imposing new sanctions.
The 2015 accord was intended to curb Iran's nuclear ambitions in return for relief from sanctions.
But its economy has been badly hit by the new sanctions policy of the Donald Trump administration. The value of Iran's currency has dropped to record lows and its annual inflation rate has quadrupled.
Battle for the fate of the Iran deal begins
Iran's Foreign Minister Javad Zarif said its latest move was within its rights under the deal. It has set a 60-day deadline for European nations to act to protect it from the US sanctions and threatened to resume the production of more highly enriched uranium if they do not do so.
Speaking on a visit to London, US Secretary of State Mike Pompeo said Iran's latest statement was "intentionally ambiguous".
Mr Pompeo said that although the US remained on the "same side" as its Western allies on countering Iranian influence, it had a different view on the nuclear deal and he indicated the US would act if trade with Iran continued beyond humanitarian aid.
He had flown to the UK from an unscheduled visit to Iraq. US officials have reported threats to US forces and their allies from Iran, but have given few details. A US aircraft carrier has been deployed to the Gulf.
Who is in the deal and what have they said?
The UK, France and Germany are still part of the deal, as are China and Russia. The US pulled out a year ago.
UK Foreign Secretary Jeremy Hunt said Iran's latest step was "unwelcome" and he urged Iran "not to take further escalatory steps" and to "stand by its commitments".
He added: "Should Iran cease to observe its nuclear commitments there will of course be consequences."
But he also said he continued to support the deal and urged Iran to think "long and hard" before it broke it.
He also admitted there was a "different approach" to that of the US, and Mr Pompeo said after talks in London that the discussion had been "forthright".
German Foreign Minister Heiko Maas said its position was to "stick by the agreement", calling it crucial for European security.
French Defence Minister Florence Parly told French media that the European powers were doing everything they could to keep the deal alive but there would be consequences and possibly sanctions if the deal was not adhered to.
Russian Foreign Minister Sergei Lavrov urged all of the remaining parties to fulfil their obligations, and said that Western participants were trying to "divert attention" from their inability to do so.
China said it "resolutely opposes" unilateral US sanctions against Iran and called for strengthened dialogue.