Russia has urged Iran to stop enriching uranium and to accept a Russian compromise aimed at defusing nuclear tensions with the West.
Western powers suspect Iran's nuclear ambitions are not peaceful
Under the plan, Tehran could receive uranium enriched on Russian soil.
But as talks began on Monday, Iran insisted again that it reserved the right to pursue nuclear research even if it accepted the Russian deal.
Western powers are concerned Iran aims to develop nuclear weapons, but Tehran says its programme is not military.
Foreign Minister Sergei Lavrov made the call for Tehran to compromise as the first of two days of talks on the issue ended in Moscow.
Iran resumed small-scale uranium enrichment earlier this month, after the UN nuclear watchdog reported Tehran to the UN Security Council.
Enrichment can produce fuel for either for civilian nuclear reactors or for nuclear bombs.
No statement was issued on the content of the Moscow talks.
But Mr Lavrov said: "The moratorium on uranium enrichment on Iranian territory should be resumed and contacts with the participation of all interested sides should continue."
Earlier, he said Moscow had only "modest expectations" from the talks.
Iran's Foreign Minister Manouchehr Mottaki, in Brussels to meet European Union officials, said nuclear research would continue.
"We continue our preparation from where we are now. That is, the research department will continue its activity," Mr Mottaki told reporters.
He said the Western threat of UN sanctions against Iran was wrong.
"We believe the time of threats is over. The Security Council should not be considered as a tool in the hands of some countries," he said.
'Hot and cold'
Russia has suggested it enrich uranium in its reactors and then ship the fuel to Iran, in a bid to alleviate the standoff.
Correspondents say Russia hopes the plan will enable Tehran to run a nuclear power programme, while allaying Western fears.
The BBC's Steve Rosenberg in Moscow says an agreement would mean more prestige for Moscow, while helping to protect its economic interests in Iran - which could be hit if the international community imposes sanctions.
However, our correspondent says Iranian officials have run hot and cold on Moscow's idea in recent weeks, and there is growing suspicion in the West that the Iranians are just stalling for time.
The International Atomic Energy Agency (IAEA) resolution could lead to eventual sanctions, although any action has been put off until a report by the head of the agency on 6 March.
The nuclear crisis has intensified since Iran resumed nuclear activity last summer after a two-and-a-half year freeze.