Updated

Iran has developed its own version of an advanced centrifuge that churns out fissile material much faster than other machines and has started testing them at the vast underground facility that houses its uranium enrichment program, diplomats and experts said Thursday.

Few of the IR-2 centrifuges were operating and testing appeared to be in an early phase with the machines rotating without processing any of the uranium gas that can either be used to generate electricity or provide the fissile core of nuclear warheads, depending on the level of enrichment.

More significant, said those with knowledge of the issue, was that Tehran has appeared to combine know-how and equipment bought on the nuclear black market with domestic ingenuity to overcome daunting technical difficulties and create advanced machines that reflect a high level of nuclear development.

Iran is under two sets of U.N. Security Council sanctions for refusing to scrap its uranium enrichment program, which it started developing during nearly two decades of covert nuclear activity built on illicit purchases and revealed only five years ago.

The council demands to freeze enrichment were prompted by concerns that it could be used to make nuclear weapons because of eroding confidence prompted by the secret nature of Iran's activities, including experiments that could be linked to the manufacture of such arms. But Tehran insists it wants to enrich only to create fuel for a future generation of reactors and insists it will expand the program instead of freezing it.