The U.N. nuclear watchdog agency held an emergency meeting Tuesday to assess Iran's resumption of uranium conversion, while an Iranian dissident said Tehran has manufactured about 4,000 centrifuges capable of enriching uranium to weapons grade.

The meeting of the International Atomic Energy Agency's (search) 35-nation board of governors came a day after Iran restarted some activities at its nuclear plant at the central Iranian city of Isfahan (search).

Alireza Jafarzadeh (search), who helped uncover nearly two decades of covert nuclear activity in 2002, told The Associated Press the centrifuges — which he said are unknown to the International Atomic Energy Agency — are ready to be installed at Iran's nuclear facility in Natanz.

Jafarzadeh, who runs Strategic Policy Consulting (search), a Washington-based think tank focusing on Iran and Iraq, said the information — which he described as "very recent" — came from sources within the Tehran regime who have proven accurate in the past.

The claims could not immediately be independently verified. The IAEA was taking the allegation "seriously" and will investigate "should we find anything credible contained within it," spokeswoman Melissa Fleming said.

In Tehran, Ali Hafezi, a spokesman for Iran's Atomic Energy Organization, told AP Tuesday that the IAEA had been given a full disclosure of Tehran's nuclear program, including the number of centrifuges. He would not say how many centrifuges Iran has.

The IAEA's board of governors was meeting to assess Iran's nuclear program and diplomats said it was likely to issue a resolution by Thursday urging Tehran to again suspend its nuclear activities.

A Western diplomat close to the agency said a draft resolution crafted by Britain, France and Germany made no mention of reporting Iran to the U.N. Security Council (search), which could impose economic or political sanctions on the regime. The diplomat spoke on condition of anonymity because he was not authorized to discuss the debate within the closed-door meetings.

IAEA chief Mohamed ElBaradei said he was hopeful the standoff could be resolved.

"I'd hope that this is simply a hiccup in the process and not a permanent rupture," ElBaradei told reporters. "We have made a very good progress in the last couple of years with regard to clarifying Iran's past nuclear activities."

Iran, which had agreed to suspend nuclear activities in November as part of negotiations, insists its nuclear program is peaceful, but Washington accuses Tehran of covertly trying to build an atomic weapon.

The work at Isfahan was resumed after IAEA inspectors installed cameras and other surveillance equipment intended to ensure no nuclear material is diverted. But ElBaradei said the surveillance equipment had not yet been tested.

Highly enriched uranium can be used to make weapons; uranium enriched to lower levels is used to produce electricity.

The agency previously had said it was aware of the existence of 164 centrifuges at Natanz, 300 miles south of Tehran.

Under an agreement with the IAEA, Iran had pledged to stop building centrifuges, which can be used to enrich uranium to levels high enough to fuel a nuclear weapon. Centrifuges also can be used for the peaceful generation of nuclear energy.

"These 4,000 centrifuge machines have not been declared to the IAEA, and the regime has kept the production of these machines hidden from the inspectors while the negotiations with the European Union have been going on over the past 21 months," Jafarzadeh said Tuesday in a telephone interview from Washington.

Jafarzadeh said Iran was making "extensive" use of front organizations or companies for the production and testing of centrifuge parts. He identified the companies as Pars Tarash, Kala Electric and Energy Novin, and said all had office space in the downtown Tehran building that houses Iran's Atomic Energy Organization.

The Iranian nuclear spokesman confirmed that the three companies work with the agency, but would not provide further details.

Britain, France and Germany have been working on behalf of the 25-nation European Union to try to persuade Iran to drop its uranium enrichment program and related activities in return for incentives. Their latest offer was rejected last weekend by Tehran.

Officials in Washington would not directly answer questions about whether the United States intends to push for sanctions now. State Department spokesman Adam Ereli did say that Iran was "thumbing its nose at a productive approach."

Daryl Kimball, executive director of the Washington-based Arms Control Association, said he thought that "the board should act by making clear that if Iran does not suspend these activities within days or a couple of weeks, they will meet again and refer the case to the Security Council."

Sending Tehran's file there now would have little effect and could even be counterproductive by encouraging nationalist sentiment, Kimball said.

Iran has insisted it has the right under the Nuclear Nonproliferation Treaty to carry out the entire fuel cycle — from raw uranium to fuel for a reactor. Europe fears that if Iran can develop fuel on its own, it will secretly produce material for a bomb.