The stern words from Moscow echo earlier remarks from the Bush administration which said Iran risked forcing the UN security council into further action.
A foreign ministry spokesman, Mikhail Kamynin, was quoted by the ITAR-TASS news agency as saying Iran's announcement "runs counter to the resolutions of the IAEA board of governors and the declarations from representatives of the United Nations Security Council."
Kamynin said Russia supported the planned visit of Mohamed ElBaradei, the head of the IAEA, to Iran on Wednesday to undertake a fact-finding mission before the April 28 deadline imposed by the UN for Iran to cease all uranium enrichment.
"We hope that Iran correctly understands the concern of the international community and takes practical steps to implement IAEA decisions," he said. "These include a call for suspension of all uranium enrichment work, including scientific research."
Confirmation from Iran that it has successfully enriched uranium will come as a blow to Russian diplomatic efforts to avert the imposition, advocated by the US, of sanctions on Iran.
"We hope that Iran correctly understands the concern of the international community and takes practical steps to implement IAEA decisions"
Mikhail Kamynin, Russian foreign ministry spokesman
Moscow had offered to aid Iran's nuclear programme by helping it to enrich uranium on Russian soil for shipment to Iranian power plants.
That offer now seems unlikely to be taken up.
Iran has traditionally regarded Russia as a main nuclear ally and several officials in Tehran have predicted that Russia will veto any punitive action by the UN security council.
The US for its part said it could not verify Iran's claims, but that even if true, it would take years before it had the capability for nuclear weapons.
White House spokesman, Scott McClellan said: "If the regime continues to move in the direction that it is currently, then we will be talking about the way forward with the other members of the Security Council and Germany about how to address this going forward."
China, which is also believed to be opposed to sanctions, urged a diplomatic solution to the row.
"We still believe that negotiations and a diplomatic solution are the best way out of it," Wang Guangya, China's envoy to the United Nations, said in New York in comments carried by the official Xinhua news agency.
It was not clear if Wang was responding directly to Iran's announcement.
Iran has said it will not back from what it says is a national right to enrich uranium to fuel nuclear power stations.
Many Western countries believe Tehran is seeking to acquire atomic warheads.
Enriched uranium can be used to
generate power or build bombs
"Based on international regulations, we will continue our path until we achieve production of industrial-scale enrichment," Mahmoud Ahmadinejad, the Iranian president, told officials and some ambassadors from regional states gathered in the northeastern city of Mashhad on Tuesday.
The level of enrichment needed to trigger the nuclear chain reaction that detonates bombs is far higher than the 3.5% Iran says it has reached.
It would take Iran years to yield enough highly enriched uranium for one bomb with its current cascade of 164 centrifuges.
But Tehran has told the IAEA it will start installing 3,000 centrifuges later this year, enough to produce material for a warhead within a year.