News agencies quote unidentified Russian sources as saying today's talks "did not manage so far to achieve a decisive breakthrough."
The two sides met at a Moscow hotel for a third round of talks after two previous negotiating sessions that made no visible progress.
The chief Iranian nuclear negotiator, Ali Larijani, was quoted by Russian news agencies as saying after the talks that Iran would not stop its own uranium enrichment, even if a deal with Russia were reached.
Larijani said talks with Moscow would be continued to work out outstanding issues, but it was not immediately clear whether negotiations would be continued tomorrow.
(AP, AFP, Reuters)