TEHRAN (Reuters) - Iran has started to install 6,000 new centrifuges at its uranium enrichment facility, President Mahmoud Ahmadinejad said on Tuesday, an expansion of nuclear work the West fears is aimed at building bombs.

Iranian President Mahmoud Ahmadinejad stands at attention during the playing of the national anthem at Tehran's Mehrabad airport, March 14, 2008. REUTERS/Raheb Homavandi

Diplomats in Vienna told Reuters last week that Tehran was installing advanced enrichment centrifuges at the underground Natanz facility, accelerating activity that could give Iran the means to make atom bombs in the future if it chose to.

Iran, the world’s fourth largest oil exporter, says it wants nuclear technology to generate electricity.

“President Ahmadinejad has announced the start of the installation of 6,000 new centrifuges at Natanz,” state radio said. State television carried a similar report.

“Today we have started the installation of 6,000 new centrifuges ... I will announce more achievements tonight,” the official IRNA news agency quoted Ahmadinejad as saying at Natanz in central Iran, surrounded by anti-aircraft guns.

The president will give a speech later on Tuesday in a ceremony in Tehran to celebrate Iran’s National Day of Nuclear Technology.

Ahmadinejad’s announcement is a new snub to the U.N. Security Council which since late 2006 has imposed three rounds of sanctions on Tehran for refusing to halt enrichment work.

A senior nuclear official told Reuters Ahmadinejad had also inspected a “new generation” of centrifuges built by Iranian scientists at a research facility at Natanz.

Washington has not ruled out military action to stop Iran’s nuclear work and Israel, Tehran’s arch-foe, has urged the international community to stop “the aggressive nuclear program of Iran”.

“The leadership (in Tehran) must understand that the international community sees the Iranian nuclear program as completely unacceptable,” said Israeli government spokesman Mark Regev in reaction to Ahmadinejad’s announcement.

Enriched uranium can be used as fuel in nuclear power plants or, if refined much more, provide material for weapons.

“FURTHER ACHIEVMENTS”

Centrifuges are machines that can spin compounds of uranium at supersonic speed to separate out and concentrate the most radioactive isotope of the element.

Analysts say they believe Iran aims to gradually replace its start-up “P-1” centrifuge with a new generation it has adapted from a “P-2” design, obtained via black markets from the West and able to enrich uranium two-three times faster than its older counterpart.

Nuclear analysts say around 1,500 centrifuges would be needed for Iran to manufacture the minimum 20 kg (45 kg) of highly enriched uranium needed for one crude warhead.

The five permanent members of the U.N. Security Council and Germany plan to meet in April to discuss whether to sweeten a 2006 offer of incentives to persuade Iran to curb its nuclear program, U.S. officials said on Monday.

A senior U.S. official made clear on Monday the Bush administration’s skepticism about improving on the offer but said it would hear out what other world powers wished to say.

Tehran has so far rejected any suggestion that it halt or limit its nuclear work in exchange for trade and other incentives, and says it will only negotiate with the U.N. nuclear watchdog.

“We have obtained our achievements under sanctions... These may delay our work but ... will encourage our scientists to continue making progress,” Ahmadinejad said.

Tehran says it believes the Western-backed push to control its nuclear program will eventually fade because of international dependence on Iranian oil and gas exports.