Iran has reached a "basic" agreement with Russia on jointly enriching uranium but there was no immediate sign today that its controversial enrichment programme at home would be suspended.
It was unclear what this basic agreement involved and both Russian and Iranian officials identified serious obstacles to a fully fledged deal.
These principally concerned a suspension of Tehran's home-grown uranium enrichment work, which many in the international community believe could be an attempt to develop nuclear weapons. Western powers are threatening to press for 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 upon its right to enrich the uranium it mines in its central desert 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," Iranian nuclear chief Gholamreza Aghazadeh told reporters in the southern Iranian port of Bushehr.
Sergei Kiriyenko, head of Russia's atomic energy agency Rosatom, 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.