The extent of the latest agreement was unclear as Russian and Iranian officials said there were still obstacles to a fully fledged deal.
"We have reached an agreement in principle for the creation of a joint company," Iran’s vice president Gholam Reza Aghazadeh said in a joint news conference with Russia's visiting atomic energy chief Sergei Kiriyenko.
Western powers have demanded that Iran suspend home-grown uranium enrichment work, or face the threat of UN sanctions.
The original Russian proposal had been for Iran's uranium to be enriched in Russia to defuse suspicions that Iran might divert some nuclear fuel into a weapons program.
However, Iran has always insisted on its right to enrich the uranium it mines on its own soil, and it was unclear how the original Russian proposal could be tailored to please Tehran.
"Regarding this joint venture, we have reached a basic agreement. Talks to complete this package will continue in coming days in Russia," Mr Aghazadeh told reporters in the southern Iranian port of Bushehr.
Mr Kiriyenko, speaking at a joint news conference with Mr Aghazadeh, said Iran still had to take "serious steps" before the deal could be completed.
He was not specific about what these steps would be, but an unnamed Russian official in Bushehr told the Interfax news agency that the deal could only go ahead if Iran suspended its own uranium enrichment. Tehran has repeatedly refused to do this.
Preconditions
Mr Aghazadeh also stipulated that Iran would be setting an unspecified "precondition" to the deal.
One EU diplomat said this precondition was almost certain to be Tehran insisting upon its right to enrich its own uranium.
"Their idea of accepting the Russian proposal is to be able to enrich in Russia and Iran, not just Russia," the diplomat told Reuters on condition of anonymity.
Europe and Washington have said they could not accept such a compromise.
Iran has been reported to the United Nations Security Council - which has the power to impose sanctions - after failing to convince the world that its nuclear ambitions are entirely peaceful. Tehran flatly denies trying to develop nuclear arms.
Konstantin Kosachev, head of the foreign relations committee in Russia's lower house of parliament, the State Duma, said the chances of an agreement were about 50-50.
"(Iran) is now using the tactic of dragging out talks as long as possible. I do not think we can expect Iran to clarify its position any time soon. I would rather suggest that this will not happen before March 6," he told Russian news agency Interfax.
March 6 is the date when the board of the UN’s watchdog, the International Atomic Energy Agency (IAEA), meets in Vienna to discuss the IAEA's latest report on Iran's nuclear program.
The report may determine whether the United States and European Union push the Security Council to impose sanctions on Iran.