The United Nations nuclear agency says Iran has started enriching uranium at an underground facility.
The International Atomic Energy Agency also says Iran has set up more than 1,000 centrifuges for enrichment at the plant at Natanz. The IAEA made the claim in a confidential document.
The enrichment process can be used to create fuel for nuclear reactors or in the creation of nuclear weapons.
Last week, Iran said it had reached an industrial scale in uranium enrichment. But nuclear experts said Iran's program was not yet at that level.
The United States and its allies have accused Iran of trying to develop atomic weapons. Iran says its nuclear program is for the peaceful production of energy.
Some information for this report provided by AFP and AP.