Iran announced a dramatic expansion of uranium enrichment Monday, saying it has begun operating 3,000 centrifuges ¡ª nearly 10 times the previously known number ¡ª in defiance of U.N. demands it halt its nuclear program or face increased sanctions.

U.S. experts say 3,000 centrifuges are in theory enough to produce a nuclear weapon, perhaps within a year. But they doubted Iran really had so many up and running, a difficult technical feat given the country's spotty success with a much smaller number.

Instead, the announcement may aim to increase support at home amid growing criticism of hardline President Mahmoud Ahmadinejad and to boost Iran's hand with the West by presenting its program as established, said Michael Levi, a nonproliferation expert at the Washington-based Council on Foreign Relations.

"From a political perspective, it's more important to have (3,000 centrifuges) in place than to have them run properly," Levi told The Associated Press. "We have an unfortunate habit to take Iran at its word when they make scary announcements."

The White House and Europe criticized the latest announcement.

"Iran continues to defy the international community and further isolate itself by expanding its nuclear program, rather than suspending uranium enrichment," said Gordon Johndroe, a spokesman for the National Security Council.

Iran is known to have had 328 centrifuges operating at its Natanz enrichment facility in central Iran. For months, it has been saying it plans to launch an expanded program of 3,000, likely to be set up in a large underground area at Natanz to protect them from air strikes.

"I declare that as of today, our dear country has joined the nuclear club of nations and can produce nuclear fuel on an industrial scale," Ahmadinejad said in a speech during a ceremony at Natanz marking the one-year anniversary of the first successful enrichment of uranium there.

His comments suggested Iran was able to produce enough enriched uranium to fuel a nuclear reactor consistently, but he did not announce the start of the 3,000 centrifuges.

Asked by reporters at the ceremony if Iran has begun injecting uranium gas into 3,000 centrifuges for enrichment, Iran's top nuclear negotiator, Ali Larijani replied, "Yes." He did not say specifically whether all were working.

In the enrichment process, uranium gas is pumped into centrifuges, which spin and purify the gas. Enriched to a low degree, the result is fuel for a reactor, but to a high degree it creates the material for a nuclear warhead.

The United States and its allies accuse Iran of seeking to develop weapons, a charge Tehran denies.

The announcement was a strong show of defiance of the United Nations, which imposed limited sanctions in December and strengthened them slightly last month because of Iran's refusal to suspend enrichment. The U.N. Security Council has set a deadline of late May for Iran to halt the program, warning it will gradually ratchet up the punishment.

Larijani warned that if the U.N. imposes further sanctions, Iran may reconsider how much it cooperates with the U.N. nuclear watchdog group under the Nuclear Nonproliferation Treaty. The International Atomic Energy Agency has been conducting inspections at Natanz and other nuclear sites.

"The European side has made the NPT ineffective by its behavior, but we are not interested in a such a thing. But when we face their harsh attitude, there is a possibility of making another decision under the pressure of the parliament," Larijani was quoted as saying on the state broadcasting company's Web site.

The Iranian parliament last year gave the government permission to reduce cooperation with the IAEA in case of sanctions.

The Vienna-based IAEA had no immediate comment on Monday's announcement.

The move showed Iran was "definitively going in the wrong direction," said the Foreign Ministry in Germany, which currently holds the European Union presidency.

U.N. Secretary-General Ban Ki-moon said he hoped Iran would "engage in dialogue. ... It is very important for any member country to fully comply with the Security Council resolution."

Larijani repeated Iran's stance that it is open to negotiations with the West and is willing to offer assurances that its program is peaceful. But he said the West must accept its nuclear program as a fact, rejecting a halt in enrichment as a precondition to talks.

So far, sanctions have been limited to a freeze of assets of some Iranian companies linked to nuclear and missile programs and a call for nations to ban travel by 15 Iranian security and government officials.

But Gen. Mohammad Baqer Zolqadr, an Iranian Revolutionary Guard general who is on the U.N. list for travel restrictions, returned on Monday from a six-day visit to Russia and boasted his trip showed "the ineffectiveness of the resolution." Moscow confirmed the visit and said it was not obliged by the sanctions resolution to ban him from travel.

The ceremonies at Natanz appeared aimed at drumming up support at home for Iran's position in the nuclear confrontation with the West. Across Iran, school bells rang on Monday to mark the "national day of nuclear energy."

Iran is also facing bitterness from Britain and the United States over its 13-day detention of 15 British sailors and marines. The 15 seized by Revolutionary Guards off the Iraqi coast were released on Wednesday. After returning home, they said they were put under psychological pressure by their captors to force them to "confess" to being in Iranian waters when captured, angering many in Britain.

The British government expressed concern over Iran's announcement Monday and called on it to suspend enrichment immediately.

"The international community stands together in ensuring that Iran does not develop the means to acquire nuclear weapons," the British Foreign Office said.

David Albright, a former U.N. nuclear inspector, said 3,000 centrifuges would be enough to build a nuclear warhead within a year.

But "it would be very hard to believe" that Iran has been able to enlarge its centrifuge cascade so dramatically, he said. "It all hinges on whether Iran will be able to get the machines working together" at a constant rate.

Levi said Iran has only been able to run its two small cascades of 164 centrifuges "perhaps 20 percent of the time."

Both Levi and Albright said 3,000 centrifuges could not produce enough fuel to keep a single reactor going.

"Iran would need approximately 25,000 of its centrifuges operating at one time to produce enough fuel for a single light water reactor," Levi said.