Iran on Monday froze sensitive nuclear work including uranium enrichment in a move likely to thwart US efforts to report it to the UN Security Council for possible sanctions. The suspension was confirmed by International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei.
"I think pretty much everything has come to a halt right now," ElBaradei told reporters in Vienna.
"We're just trying to apply seals and make sure everything has been stopped," he said.
Operations at the uranium conversion facility at Isfahan had also stopped, he said.
The suspension of processes that could be used to develop nuclear weapons came into effect on Monday - just three days ahead of a meeting of the IAEA board to discuss Iran's case.
Washington accuses Iran of developing nuclear weapons. It wants the IAEA board to refer Iran to the Security Council and sanctions imposed on Tehran for failing to disclose a range of nuclear work and facilities in the past.
But Iran - which says its nuclear programme is geared entirely to electricity generation, not bombs - last week struck a deal with the European Union to avoid Security Council referral if it suspended all sensitive nuclear work.
Tehran has said that this suspension would be short-lived.
However, ElBaradei urged the Iranians to maintain it for the duration of the IAEA inspection process.
"I think it's in Iran's interest to maintain the suspension while we are going through the (inspections), while we are trying to restore confidence that the programme is for peaceful purposes," he said.
The EU hopes to persuade Iran in the coming months to make the temporary suspension permanent by offering it a range of economic and political incentives.
But it may still refer Iran to the Security Council if it goes back on the deal, British Foreign Secretary Jack Straw said.
Washington is sceptical that Iran will stick to any deal and Iran says it will never abandon its nuclear programme.
Scepticism about Iran's intentions was heightened last week when Iranian exiles accused Tehran of running a secret enrichment plant and diplomats revealed that Iran had stepped up some uranium conversion activities just ahead of the suspension.