Iran has postponed talks with Russia on a proposal to enrich uranium on Russian soil, amid reports that Tehran has resumed its own small-scale enrichment.
Thursday's talks have been pushed back indefinitely, Iranian presidential spokesman Gholamhossein Elham said.
He said the talks had been postponed because of the "new situation".
Earlier this month, the UN nuclear watchdog, the IAEA, voted to report Tehran to the Security Council over its decision to resume nuclear research.
According to diplomats in Vienna, home of the International Atomic Energy Agency's headquarters, workers at Iran's Natanz nuclear facility have begun putting uranium hexafluoride (UF6) gas into centrifuges used to distil enriched uranium.
Iranian officials had warned they would restart small-scale uranium enrichment by early March, but they did not specify a date.
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Western powers are concerned that Iran's decision to resume research into uranium enrichment - a process which creates fuel for nuclear reactors and, potentially, for a nuclear bomb - is part of a plan to acquire nuclear weaponry.
Iran says its programme is solely aimed at energy production.
As a means to alleviate the standoff, Russia had proposed that it enrich the uranium on its reactors and then ship the fuel to Iran.
Russia supported the IAEA's decision to refer Iran to the UN Security Council.
Talks with Moscow will recommence at a time of "mutual agreement", Mr Elham said.
For its part, Russia says talks could still take place this week.
"Our offer for the 16th still stands," Russian Deputy Foreign Minister Sergei Kislyak said.
Nuclear treaty withdrawal
After Tehran was reported to the Security Council on 4 February, Iran announced it would end its voluntary freeze on full-scale uranium enrichment and would stop allowing snap UN inspections of its nuclear sites.
The referral, which could lead to sanctions, has been roundly condemned in Tehran, and Iranian President Mahmoud Ahmadinejad is now threatening to quit the Non-Proliferation Treaty (NPT).
In a speech marking the anniversary of the 1979 Islamic revolution on Saturday Mr Ahmadinejad said that if the rights of the Iranian people were violated, Iran would "revise its policies".
The NPT, which has 187 signatories, was created to prevent new nuclear states emerging, to promote co-operation in the peaceful use of nuclear energy and to work towards nuclear disarmament.
Non-nuclear signatories agree not to seek to develop or acquire such weapons. In return, they are given an undertaking that they will be helped to develop nuclear energy for peaceful purposes.
It is believed to be the first time Iran has threatened to pull out of the treaty.
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