The United Nations nuclear watchdog has confirmed that Iran has started enriching uranium at an underground site and said all atomic material there was under its surveillance.
Diplomats in Vienna, home of the UN's International Atomic Energy Agency (IAEA), earlier said Iran had begun refining uranium to a fissile purity of 20 percent at Fordow near the Shi'ite Muslim holy city of Qom.
In Washington, U.S. State Department spokeswoman Victoria Nuland called Iran's uranium enrichment work at the new site a "further escalation" in the nuclear showdown with the international community.
Washington and its allies say Iran is using its nuclear program as a cover to develop atomic bombs, and the United States and the European Union recently expanded sanctions against Iran over its nuclear program.
Iran denies this but so far has refused to halt uranium enrichment as demanded by the UN Security Council.
"The IAEA can confirm that Iran has started the production of uranium enriched up to 20 percent...using IR-1 centrifuges in the Fordow Fuel Enrichment Plant," agency spokeswoman Gill Tudor said in an e-mail.
"All nuclear material in the facility remains under the agency's containment and surveillance," she said.
Earlier, Iran's Supreme Leader Ayatollah Ali Khamenei, in a speech broadcast on state television, said further sanctions imposed by the West would not make Iran change its nuclear course.
compiled from agency reports