Iranian and Russian officials have ended two days of talks on a Russian plan to enrich uranium on its territory for use in Iran's reactors.
Western powers suspect Iran's nuclear ambitions are not peaceful
It is not clear what the talks may have achieved as the Iranian delegation prepared to leave Moscow on Tuesday.
A senior Iranian official described the talks as "positive and constructive" and said they would continue.
Russian Foreign Minister Sergey Lavrov said it was too early to declare the talks a failure.
Russian atomic agency head Sergei Kiriyenko is expected to visit Iran on Thursday for further discussions.
Separately, Iranian Foreign Minister Manouchehr Mottaki said Tehran would no longer hold nuclear talks with the EU-3 of Britain, France and Germany.
He said Iran attached greater weight to negotiations with Moscow on a Russian compromise plan.
"Our contacts with the European Union will no longer be held with the EU-3, but with the different countries of the European Union," Mr Mottaki said a day after talks with EU officials in Brussels.
The Russian proposal was being seen by many as a last chance for Iran to compromise with the UN nuclear agency, the International Atomic Energy Agency.
On 6 March the agency is due to issue a report that might move the whole issue to the UN Security Council.
Iran has previously insisted that it will not give up the right to enrich uranium on its own territory.
The agency reported Iran to the security council in January over a lack of co-operation and transparency in its nuclear activities.
Western powers are concerned Iran aims to develop nuclear weapons, but Tehran says its programme is not military.
Iran resumed small-scale uranium enrichment earlier this month.
Enrichment can produce fuel for either civilian nuclear reactors or nuclear bombs.
The nuclear crisis has intensified since Iran resumed nuclear activity last summer after a two-and-a-half year freeze.