If confirmed, the announcement would be a major expansion of Iran's uranium enrichment - a process that can produce either fuel for a nuclear reactor or material for a warhead.
U.S. Secretary of State Condoleezza Rice, however, cautioned that the claim could not be immediately substantiated. Diplomats close to the U.N. nuclear watchdog agency said Iran has exaggerated its enrichment advance, and has had problems operating the 3,000 centrifuges it already has in place. One diplomat cast doubt on Ahmadinejad's claims of a more advanced centrifuge.
Permanent members of the U.N. Security Council, which has already imposed three sets of sanctions against Iran for its refusal to suspend uranium enrichment, were divided on how to respond.
The United States and Britain quickly condemned the announcement, and France warned Iran could face more sanctions. But Russia, an ally of Iran, dismissed the need for that, saying negotiators were preparing a new package of incentives aimed at persuading Iran to freeze uranium enrichment.
Iran rejected one European package of incentives last week. Tehran says its nuclear program is intended only to produce energy, not develop weapons as the U.S. and many of its allies fear.
Iran already has about 3,000 centrifuges operating at its underground nuclear facility in Natanz. A total of 3,000 centrifuges is the commonly accepted figure for a nuclear enrichment program that is past the experimental stage and can be used as a platform for a full industrial-scale program that could churn out enough enriched material for dozens of nuclear weapons over time.
Ahmadinejad toured the Natanz facility in ceremonies marking the second anniversary of the day Iran first enriched uranium in 2006.
During the tour, he announced the start of work on installing the 6,000 new centrifuges. Later in a nationally televised speech, he announced the testing of the new, more effective centrifuge.
Ahmadinejad said a "new machine was put to test" that is smaller but five times more efficient than the P-1 centrifuges that are currently in operation at Natanz. He provided no further details on the new device or on how many Iran had.
He called the development a "breakthrough" and the "beginning of a speedy trend to eliminate the big powers" dominance in nuclear energy.
He lauded Iran's achieved proficiency in the cycle of nuclear fuel despite U.N. sanctions and pressures imposed by the world's big powers.
But a diplomat following Iran's nuclear program at the Vienna, Austria-based International Atomic Energy Agency, said Ahmadinejad's statement appeared to be "a lot of sound and fury signifying nothing."
"It seems to be little more than a publicity stunt," said the diplomat, who spoke on condition of anonymity because he was not authorized comment publicly.
The U.N. has passed three sets of sanctions against Iran for its refusal to suspend uranium enrichment.
French Foreign Minister Bernard Kouchner told reporters in Paris that if the reports of 6,000 more centrifuges were true, the international community "must reinforce sanctions" even as it continues to pursue dialogue.
However, Russia's Foreign Minister Sergey Lavrov said there was no need for new sanctions. Instead, he told Ekho Moskvy radio that diplomats from the U.S., Russia, China, Britain, France and Germany, would offer Iran new economic, energy and security incentives to halt uranium enrichment.
Rice urged Iran to accept a deal and halt enrichment.
"Iran faces continued isolation in the international community because it will not take a reasonable offer from the international community to have another way," she said in Washington. "The six parties have put forward, I think, a very generous set of incentives should Iran agree to live up to the obligations that any state has when a Security Council resolution is passed."
Gregory Schulte, the U.S. representative to the Vienna-based International Atomic Energy Agency, said the Iran announcement "reflects the Iranian leadership's continuing violation of international obligations and refusal to address international concerns."
"This approach has not brought Iran international respect or accolade but rather increasing censure and sanction," he said in a written statement.
Britain's Foreign Office said Iran has "chosen to ignore the will of the international community," accusing Tehran of "making no effort to restore international confidence in its intentions."
In the enrichment process, uranium gas is pumped into series of centrifuges called "cascades." The gas is spun at supersonic speeds to remove impurities. Enriching at a low level produces nuclear fuel, but at a higher level it can produce the material for a warhead.
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 more advanced IR-2 centrifuge, which can churn out enriched uranium at more than double the rate.
It was not clear if the new centrifuge Ahmadinejad spoke of was the IR-2 and Iranian state television, which carried the president's speech in a live broadcast, didn't say if the installation of the 6,000 new centrifuges included the older P-1 or the advanced IR-2 centrifuges.
Iran says it plans to move toward large-scale uranium enrichment that ultimately will involve 54,000 centrifuges.