Russian Foreign Minister Sergei Lavrov says his country has no evidence Iran has made the advances needed to produce nuclear fuel on an industrial scale.
Visitors to the Natanz plant were not shown the 3,000 centrifuges
On Monday, Iran said it could produce nuclear fuel on such a scale.
Earlier, the head of Iran's atomic energy agency said his country intended to continue enlarging its capacity to produce nuclear fuel.
Iran says its nuclear programme is peaceful, but the West fears it wants to build atomic bombs.
Mr Lavrov said Russia treated the announcement seriously, but Moscow had no confirmation that uranium enrichment had begun in new centrifuges - the machines which spin uranium gas to enrich it - which Iran says it has installed at its Natanz plant.
"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.
Earlier on Tuesday, Gholam Reza Aghazadeh, the head of Iran's atomic energy agency, said Iran planned to continue installing some 50,000 centrifuges at the uranium enrichment facility at Natanz, the state news agency reported.
The United States expressed deep concern over the news.
"Iran continues to defy the international community and further isolate itself by expanding its nuclear programme," a White House spokesman said.
There was similar condemnation from the United Nations and European Union.
Package of sanctions
The UN has passed two packages of sanctions against Iran for refusing to suspend its uranium enrichment programme.
There is no independent confirmation that Iran has reached its initial target of installing 3,000 centrifuges.
However, previous Iranian claims have proved true when international inspectors have had access.
Journalists and diplomats visiting the Natanz plant were not shown the centrifuges. These are underground to protect them from possible military attack, while the site is surrounded by anti aircraft guns.
On Monday, Ali Larijani, Iran's chief nuclear negotiator with the West, warned that Iran would have no choice but to review its membership of the Nuclear Non-Proliferation Treaty if further pressure was applied by the West over its nuclear programme.