Russia today questioned an announcement by Iran that it was now making nuclear fuel on an industrial scale.
If confirmed, the move would take Tehran closer to making an atomic bomb - which the West fears is its aim.
Two UN inspectors, who could provide the first independent confirmation of any Iranian progress, arrived today to inspect the Natanz uranium enrichment site, where President Mahmoud Ahmadinejad said Iran had expanded its atomic work.
Iran's enrichment activity, until now at an experimental level, has drawn international criticism, including from Russia, its closest big power ally. The UN Security Council has imposed sanctions on Iran for not stopping the work.
"We are not aware of any technological breakthroughs in the Iranian nuclear programme recently which would change the nature of work on enrichment being carried out in the country," Russia's Foreign Ministry said in a statement.
Western analysts say Iran has made grand claims in the past about progress to strengthen its bargaining hand with the West but say Tehran has glossed over technical glitches that mean it is several years from being able to make a bomb.
Iran, the world's fourth largest oil exporter, insists it wants only to make fuel for atomic power plants it is planning. Its first one is still under construction with Russian help.
The semi-official Fars news agency said the International Atomic Energy Agency (IAEA)
inspectors would stay in Iran for one week. An Iranian official said they were on a routine visit.
Inspectors from IAEA routinely visit Natanz and other atomic sites, but Tehran halted more intrusive snap checks last year when its case was sent to the UN Security Council.
Tensions over the nuclear row have underpinned oil prices and helped them higher today towards $62 a barrel.