VIENNA, Austria – Iran is delivering small amounts of uranium gas to centrifuges that can enrich it to weapons-grade level and is running more than 1,300 centrifuge machines, according to an International Atomic Energy Agency document.

The confidential document – a letter to Iranian officials from a senior staff member at the International Atomic Energy Agency – also protests an Iranian decision to prevent agency inspectors from visiting the country’s heavy water facility that, when built, will produce plutonium.

Enriched uranium and plutonium can both be used for the fissile core of nuclear warheads.

The letter, signed by IAEA deputy director general Olli Heinonen and dated April 18, said Iran has provided information to the agency that it has put into operation 1,312 centrifuges – the machines used to spin the gas into enriched uranium.

The letter also cites Iranian information to the agency that “some UF6 is being fed” into the centrifuges at the underground Natanz facility, referring to the uranium gas that can be enriched to levels potent enough to be used for nuclear arms.

Iran stopped experimental enrichment – which it was doing on a much smaller scale – in exchange for negotiations with European nations. Talks broke down in 2005, but Tehran has generally refrained from even small-scale enrichment, while continuing to develop the technology.

The document reports a significant development, particularly considering the number of centrifuges involved, and the next step – large-scale enrichment.

Iran says it wants to enrich uranium only to lower levels suitable to generate nuclear power. But suspicions about its ultimate intentions have led to U.N. Security Council sanctions for its refusal to freeze its enrichment program.

Last week, Iran said it had begun operating 3,000 centrifuges at Natanz – nearly 10 times the previously known number. The U.S., Britain, France and others criticized the announcement, but experts – and several world powers – expressed skepticism that Iran’s claims were true.

Still, the IAEA letter reflected a swift advance in the program. A little more than two weeks ago, diplomats familiar with Iran’s nuclear dossier had said Teheran was running just more than 600 centrifuges, and had not introduced any uranium gas into any of them.

It was unclear what the purpose of the uranium gas feed was. A diplomat accredited to the IAEA, who demanded anonymity because he was disclosing confidential information, said the operation appeared to be part of preparing the centrifuges for producing enriched uranium, and not yet part of the direct enrichment process, although traces of low-enriched uranium were being produced.

Another diplomat said that the process appeared to be last step before larger-scale enrichment begins.

Experts say smooth operation of 3,000 centrifuges would make enough material for a nuclear warhead within a year. But Iranian officials recently acknowledged that 10 to 20 percent of the centrifuges were breaking down in test runs – a rate one of the Vienna-based diplomats said was likely fairly accurate.