President Mahmoud Ahmadinejad said Saturday that Iran now possesses 6,000 centrifuges, a significant increase in the number of uranium-enriching machines in its nuclear program, the semi-official Fars news agency reported.

The new figure is double the 3,000 centrifuges Iran had previously said it was operating in its uranium enrichment plant in Natanz.

"Islamic Iran today possesses 6,000 centrifuges," Ahmadinejad told university professors in the northeastern city of Mashhad.

In April, Ahmadinejad said Iran had begun installing 6,000 centrifuges at Natanz. His reported comments Saturday provided the first public assertion that Iran has reached that goal.

The announcement is another act of defiance in the face of demands by the United States and other world powers for Iran to halt its enrichment work, which Washington and its allies fear Iran is intent on using to develop weapons.

Ahmadinejad said those nations — the U.S., Britain, France, Russia, China and Germany — have tempered their demands, asking Iran not to freeze enrichment but rather not to expand its current program beyond 6,000 centrifuges, state-run radio reported.

"Today, they have consented that the existing 5,000 or 6,000 centrifuges not be increased and that operation of this number of centrifuges is not a problem," Ahmadinejad said.

In its negotiation with Iran, the five permanent members of the U.N. Security Council plus Germany have offered a package of technological, economic and political incentives in return for Iran's cooperation.

A report by the U.N.'s nuclear monitoring agency delivered to the Security Council in May said Iran had 3,500 centrifuges, although a senior U.N. official said at the time that Iran's goal of 6,000 machines running by the summer was "pretty much plausible."

Uranium can be used as nuclear reactor fuel or as the core for atomic warheads, depending on the degree of enrichment. Iran says it is interested in enrichment only for its nuclear power program.

The workhorse of Iran's enrichment program is the P-1 centrifuge, which is run in cascades of 164 machines. But Iranian officials confirmed in February that they had started using the IR-2 centrifuge that can churn out enriched uranium at more than double the rate.

A total of 3,000 centrifuges is the commonly accepted figure for a nuclear enrichment program that surpasses the experimental stage and can be used as a platform for a full industrial-scale program that could churn out enough material for dozens of nuclear weapons.

Iran says it plans to move toward large-scale uranium enrichment that ultimately will involve 54,000 centrifuges.