Iran has signed an agreement to send uranium abroad for enrichment after mediation talks in Tehran with Turkish and Brazilian leaders.
Correspondents say the plan could revive a UN-backed proposal and may ward off another round of sanctions.
But the BBC's Tehran correspondent says the deal will be viewed with scepticism in Western capitals, as Iran says it will continue enriching uranium.
Meanwhile, France has announced progress at the UN on fresh sanctions.
AFP news agency reported that Israeli officials were accusing Iran of manipulating Brazil and Turkey to stave off sanctions.
The West has long suspected that Iran's nuclear programme is aimed at making weapons - a charge Tehran denies.
Under the deal, Iran's foreign ministry said it was ready to ship 1,200kg (2,645lb) of low-enriched uranium to Turkey, in return for fuel for a research reactor.
The deal does not address the central nuclear issues dealt with by successive UN Security Council resolutions - namely Iran's refusal to halt its enrichment programme and address questions about its past nuclear activities.
The US reacted by saying it still had serious concerns over Iran's nuclear programme, although it did not reject the agreement.
It said the Iranian government "must demonstrate through deeds - and not simply words - its willingness to live up to international obligations or face consequences, including sanctions".
"While it would be a positive step for Iran to transfer low-enriched uranium off of its soil as it agreed to do last October, Iran said today that it would continue its 20% enrichment, which is a direct violation of UN Security Council resolutions," said a White House statement.
Russia welcomed the deal, although President Dmitry Medvedev said further talks were needed on Iran's nuclear programme.
During a trip to Ukraine Mr Medvedev said the fact that Iran apparently still intended to continue its own uranium enrichment would continue to concern the international community.
French Foreign Minister Bernard Kouchner said there had been "some important progress" in talks at the Security Council on fresh sanctions against Tehran.
The UK, for its part, said work on a resolution about imposing new sanctions on Iran would continue until Tehran showed its intentions were peaceful.
The German government said that nothing could replace a deal between Iran and the UN's nuclear watchdog, the IAEA.
Iran's President Mahmoud Ahmadinejad called on world leaders for new talks "with Iran based on honesty, justice and mutual respect".
The EU's high representative for foreign affairs, Baroness Ashton, was ready to meet the Iranian authorities to find a "full and complete" solution to the stand-off, her spokesman said.
Turkish Prime Minister Recep Tayyip Erdogan and Brazil's President Luiz Inacio Lula da Silva were at the talks in Tehran with Mr Ahmadinejad.
Iranian foreign ministry spokesman Ramin Mehmanparast said 1,200kg of low-enriched uranium would be shipped to Turkey, and that Iran would notify the IAEA "within a week".
Under the deal, Iran has said it is prepared to move its uranium within a month of its approval by the so-called Vienna Group (US, Russia, France and the IAEA).
In return, Iran says it expects to receive 120kg of more highly enriched uranium (20%) - a purity well below that used in the manufacture of nuclear weapons - within a year.
If the deadline is not met, Iran says Turkey "will return swiftly and unconditionally Iran's low-enriched uranium".
BBC Tehran correspondent Jon Leyne, in London, says the agreement does not make clear whether Iran's low-grade uranium will be used to make the new fuel or just held as a kind of security deposit.
Our correspondent says Western governments will fear this is just a negotiating ploy designed to delay new sanctions.
Crucially, Turkey and Brazil are both on the UN Security Council, and so have a vote on those sanctions.
The Turkish Foreign Minister, Ahmet Davutoglu, who spent 18 hours hammering out the deal with his Brazilian and Iranian counterparts, said there was now no need for more sanctions against Iran.
"The swap deal shows that Tehran wants to open a constructive path... there is no more ground for new sanctions and pressures," he said.
The US is in the final stages of negotiating a fourth sanctions package with other UN Security Council members.
This new deal will be examined in great detail and with a high degree of scepticism in foreign capitals, our correspondent says.
Iran backed out of a similar proposal last October citing disagreement about the details of the deal, which included a simultaneous swap, something the IAEA said was not feasible.
Iran's stocks are now thought to be much larger than the 1,200kg covered by the new agreement.
Both Russia and the US say the talks represent Iran's last chance to avoid harsher sanctions.
Iran has been mounting a big diplomatic effort to prevent new UN sanctions; its foreign minister has visited all 15 members of the Security Council.