Iran has begun the installation of 6,000 advanced centrifuges in its Natanz uranium enrichment facility, according to state media reports quoting the president.
The announcement on Tuesday marks Iran's "national day of nuclear technology", commemorating the day in 2006 when Iran first produced uranium sufficiently enriched to make atomic fuel.
"Today, the phase for installing 6,000 new centrifuges at the facility in Natanz has started," the website of Iran's state broadcaster quoted Mahmoud Ahmadinejad, the president, as saying at the underground nuclear facility.
Iran already has about 3,000 centrifuges operating in Natanz.
Ahmadinejad made Tuesday's announcement as he toured the Natanz facility in central Iran.
Iranian television also quoted him as saying that "other activities have been carried out" in Natanz that he would announce later on Tuesday.
Sanctions
The announcement of the new reactors is seen by many as a show of defiance to international demands that Iran halt its nuclear programme, which the US and others say is aimed at building nuclear weapons.
The UN Security Council has, since late 2006, imposed three rounds of sanctions on Tehran for refusing to halt its enrichment work.
Iran says the sanctions, targeting its nuclear and military sectors, as well as its financial transactions with the outside world, have not harmed the state.
The five permanent members of the Security Council and Germany plan to meet in April to discuss whether to broaden a 2006 offer of incentives to persuade Iran to curb its nuclear programme, US officials said on Monday.
Tehran has so far rejected any suggestion that it halt or limit its nuclear work in exchange for trade and other incentives, and says it will negotiate only with the UN nuclear watchdog, the International Atomic Energy Agency.