The negotiations on Moscow's offer to carry out ultra-sensitive uranium enrichment work on Russian soil are seen as a last chance for Tehran to avoid being hauled before the UN Security Council for possible punitive action.
Having already been reported to the Security Council, Iran is also under pressure to provide greater access to International Atomic Energy Agency (IAEA) inspectors and return to a full freeze of enrichment work, which can be extended to weapons making.
But the tone in Tehran was defiant. "The Islamic republic is not being stubborn or adventurist, but it will not accept the suppression of its nuclear fuel activities," said Javad Vaidi, a member of Iran's Supreme National Security Council.
"A suppression of our enrichment activities, as demanded by the West, would be synonymous with humiliation and national dependence ... and we will not accept it," he told Iranian media.
EU talks
Mr Vaidi is also travelling to Brussels with Iranian Foreign Minister
Manouchehr Mottaki for talks on Monday with European Union foreign policy chief Javier Solana.
National security official Ali Hosseini-Tash, who will be leading the
Iranian team to Moscow, told state television that "the Islamic republic's officials have said they will not back down in defending their rights".
Russia has been hoping the idea will satisfy Western objections to Iran holding technology that can be extended to make weapons, while at the same time providing the Islamic republic with fuel for its nuclear energy drive.
But Iran says it only wants to make electricity and that its right to enrich uranium for fuel is therefore enshrined under the nuclear Non-Proliferation Treaty.
Mr Mottaki also said other countries must "accept fuel making in Iran in accordance with the NPT."
"The Russian plan needs more discussions and clarifications, notably on who can participate in the plan, its calendar and on the places where enrichment can take place," Mottaki was quoted as saying by the official news agency IRNA.
There have been growing doubts Iran would accept the Russian compromise following its decision to restart its own small-scale uranium enrichment while at the same time mobilising public opinion in a way that leaves little room for compromise.
Western pressure
Iran could defuse international concerns over its nuclear program by following the example of Libya, the US ambassador to the UN, John Bolton, said in an interview published in Time magazine.
Libya renounced weapons of mass destruction in 2004, leading to a rapprochement between Washington and Tripoli after 24 years of isolation and sanctions.
Asked if the dispute over Iran's nuclear ambitions could be resolved through diplomacy, Mr Bolton said: "Sure. I never would have guessed that Libya was prepared to make the calculation that they were safer giving up the pursuit of nuclear weapons than continuing to go after them, and yet they did (give them up).
"And that led to substantial progress in the relationship between Libya and the United States. If Libya can do it, Iran can do it too.
"That's why I say the decision ultimately is largely in their hands," Mr Bolton said.
Opinion has been hardening in the West, with US Secretary of State
Condoleezza Rice this week branding Iran's government "a strategic challenge to the United States, to the world, and a destabilising influence in the Middle East".
France's Foreign Minister Philippe Douste-Blazy has accused Iran of harbouring "a clandestine, military" project.
If Iran rejects the Russian plan, tension will rise rapidly ahead of the
March 6 meeting of the Security Council, possibly opening the way to a debate on sanctions, an escalation that analysts say could have unpredictable and dangerous consequences.