President Mahmoud Ahmadinejad said Thursday Iran has produced the first stock of uranium enriched to 20 percent for a research reactor in Tehran.

"We have produced the first stock of 20 percent enriched uranium at the Natanz enrichment facility," Ahmadinejad told a huge crowd of people in Tehran's Azadi Square.

The crowd had gathered to commemorate the 31st anniversary of the Islamic Revolution. Millions of people attended the rally to celebrate the overthrowing of the Shah regime, a US-backed monarchy.

In his address, President Ahmadinejad said the nuclear program remains under the full supervision of the UN nuclear watchdog, adding that the country has the capability to enrich uranium to levels higher than 20 percent.

Iran says the program is aimed at the civilian applications of the technology. The West, however, accuses the country of seeking to develop nuclear weaponry.

In an allusion to the allegation, President Ahmadinejad said building an atomic bomb is impossible under UN supervision.

"Nuclear bombs will be built by those who are not signatories to the [Nuclear Non-Proliferation Treaty] NPT," he said, accusing the West of nuclear double standards.

"We do not seek nuclear weapons. We are brave enough to admit it."

The remarks come as the Obama administration has stepped up its anti-Iran rhetoric, calling on its allies to back a White House initiative to impose fresh sanctions against the country over a delayed nuclear fuel swap deal.

Under the proposal, Iran sought to buy the fuel required for the research reactor in Tehran, which produces medical isotopes. However, West's inflexibility in negotiations prompted President Ahmadinejad to decide to take measures to produce the fuel domestically.