Officials in the United States and at the United Nations have criticized Iran's announcement that it has expanded its nuclear program and can now enrich uranium on an "industrial scale."
The European Union called the Iranian announcement a step in "the wrong direction."
Russia expressed skepticism, with Foreign Minister Sergei Lavrov saying that Moscow has no information to confirm Iran's claim.
Iran's top nuclear negotiator, Ali Larijani, said Monday that Iran has begun to feed uranium gas into 3,000 centrifuges for enrichment - a major expansion of the process.
U.N. Secretary-General Ban Ki-moon responded by urging Iran to obey Security Council demands to halt enrichment of uranium - which can be used to produce nuclear weapons.
A U.S. State Department spokesman said the announcement shows Iran's continued defiance of the international community. He said it also shows that U.N. action against Iran is legitimate and justified.
The U.N. Security Council has passed two resolutions imposing sanctions on Iran since December for its refusal to suspend uranium enrichment.
Iranian nuclear negotiator Larijani also said that Iran would have to reconsider its membership in the nuclear Non-Proliferation Treaty - if it is subjected to further pressure over its nuclear program.
The United States and its allies accuse Iran of trying to develop nuclear weapons. Iran says its nuclear program is for the peaceful production of energy.