European and world leaders did not mince words after the claim, which was made by Iranian President Mahmoud Ahmadinejad on Monday at Iran's Natanz nuclear facility.

Germany's Foreign Ministry said Tuesday that Ahmadinejad's plans were in direct contradiction with two resolutions made by the United Nations Security Council and were of "great concern."

"Such a step would be a direct contradiction of the repeated requests by the International Atomic Energy Agency's board of governors and of the binding demands of the UN Security Council's resolutions 1737 and 1747 on Iran to cease all enrichment activities," a ministry statement read.

Currently president of both the European Union and Group of Eight industrialized nations, Berlin also called on Iran to return to negotiations concerning its nuclear program.

The foreign policy spokesman for the government's conservative faction in Germany, Eckart von Klaeden, said that NATO should intensify its discussions about security following the announcement.

"In my opinion, Iran is the biggest danger to world peace at the moment," Klaeden said in a radio interview.

Russia, which has previously helped Iran in developing nuclear technology, echoed those sentiments. Konstantin Kosachov, the chairman of the Foreign Relations Committee in the Russian parliament, called Iran's announcement "a direct challenge to the world community."

A British Foreign Office spokesman added that the international community "stands together in ensuring that Iran does not develop the means to acquire nuclear weapons."

UN Secretary General Ban Ki-Moon and the United States have also expressed serious concerns at Iran's insistence on expanding its nuclear program.

Tehran remains defiant

But the flurry of warnings has had little effect on Iran's leaders. On Tuesday, the head of Iran's atomic energy organization, Gholam Reza Aghazadeh, said the country would drastically increase its number of centrifuges, which are essential in the enrichment of uranium.

"The objective of the Islamic Republic of Iran is not just the installation of 3,000 centrifuges at the Natanz plant," Aghazadeh said. "We are doing everything to install 50,000 centrifuges."

Iran insists that its nuclear program is aimed at producing energy, not weapons, and says that it has the right to enrich uranium under the terms of the Nuclear Non-Proliferation Treaty. But the US and many European countries suspect Tehran is secretly trying to develop nuclear weapons.

Enriched uranium is used for producing nuclear fuel, but if the level of enrichment is increased it can be used to build weaponry.

Scepticism about claims

But there are questions as to whether Iran truly has achieved the sort of major breakthrough it announced on Monday. The Russian Foreign Ministry said it has no evidence to support Tehran's claims.

And American experts also expressed strong doubts as to whether Iran had really mastered the highly complicated process of operating so many centrifuges simultaneously.

"I don't believe they have 3,000 up and running in any reasonable sense," said Michael Levi, a non-proliferation expert at the Washington-based Council on Foreign Relations.

The UN's nuclear watchdog, the International Nuclear Energy Agency has said some 60,000 centrifuges are needed to operate an industrial-scale uranium enrichment for fuel production. Two UN inspectors have arrived in Natanz for a week-long visit to the facility.