Moscow is offering to enrich the uranium Iran needs for nuclear power stations on Russian soil, an arrangement that would help ease international concerns Tehran could divert the material for bomb-making.
The Russian offer, backed by the United States and Europe, represents an eleventh-hour chance for Iran to address concerns before a 6 March meeting of the International Atomic Energy Agency (IAEA), which could start a process leading to punishment.
The arrival of the Iranians was kept low-key, with no Russian officials turning up at Moscow's Sheremetyevo airport to meet the delegation, led by Ali Hosseinitash, a deputy head of the Supreme National Security Council.
Members of the delegation declined to comment on the format or contents of their planned talks with counterparts from Russia's Security Council and its nuclear agency Rosatom.
Rose Gottemoeller, the director of the Carnegie Moscow Centre, said: "At the moment there's only one diplomatic door left open, and it's open a crack.
"At the moment there's only one diplomatic door left open, and it's open a crack"
Director, Carnegie Moscow Centre
"So I think this set of talks on Monday is very important for the future of the diplomatic approach."
But privately Western diplomats are sceptical Tehran will accept the proposal.
Iran says it has a sovereign right to carry out a full nuclear fuel cycle on its own soil, but is willing to hear more about the Russian plan. Western diplomats say they believe Tehran is keeping the Russian offer on the table to buy time.
"It's just window-dressing. We don't see the Iranians as serious about the proposal," one EU3 diplomat, who requested anonymity, told Reuters.
European officials will also make a fresh appeal to Iran to halt sensitive nuclear activities when Manouchehr Mottaki, the Iranian Foreign Minister, visits Brussels on Monday.
Mottaki told reporters at Tehran's Mehrabad airport on Sunday that he was optimistic about the talks.
Iran says it is willing to hear out
the Russian proposal
"We are ready to hear any new plan ... provided Iran's right to uranium enrichment is preserved," he said.
Countries on the board of the IAEA, have called for the Iranian controversy to be referred to the UN Security Council by 6 March.
The Security Council, where Russia has a veto, has the power to impose sanctions.