"Why should Iran suspend its research activities?" Ali Larijani, the head of the Supreme National Security Council, was quoted as saying by the official IRNA news agency.

Enrichment can be extended from making reactor fuel to the production of warheads, but Larijani branded a UN Security Council demand for a suspension by April 28 as "not rational".

"One should not follow such propositions... which are not rational," he said, adding: "Iran will follow its nuclear programme with patience."

Last Tuesday Iran announced it had successfully enriched uranium to the level needed for reactor fuel, reigniting fears that the hardline regime would soon acquire the technical know-how to make bombs.

The deputy head of Iran's Atomic Energy Organisation, Mohammad Saidi, also argued that the UN nuclear watchdog had failed to find any proof that Iran's programme was anything other than a legal effort to generate electricity.

"Therefore there is no need to continue a suspension," he told the Etemad-Melli newspaper.

"These countries have to accept the reality and realise they are talking with a country that masters this technology and wishes to develop it," Saidi said.

The five permanent members of the Council -- Britain, China, France, Russia and the United States -- plus Germany meet in Moscow Tuesday to discuss the issue amid a US push for robust UN action.

In Tokyo, a foreign ministry official also said Japan will send a senior envoy to Iran by the end of the month to urge it to stop uranium enrichment -- following up on similar efforts by China and International Atomic Energy Agency chief Mohamed ElBaradei.

"We advise them not to repeat past mistakes... so that a reasonable atmosphere is created to follow up the negotiations," Larijani said of the suspension demands.

He also dismissed a proposal by US Secretary of State Condoleezza Rice, who has called on the Security Council to adopt a resolution which could allow the use of force against Iran.

"Such statements are not new and will not affect our determination," he said.

Larijani also implicitly confirmed comments by hardline President Mahmoud Ahmadinejad that Iran was working on using advanced P-2 centrifuges -- devices that can enrich at a much faster rate than the existing P-1 technology Iran is presently using. When asked about the new work, Larijani replied that Iran "will continue research work within the framework" of the IAEA.

According to the New York Times, Ahmadinejad's revelation of the P-2 work has provoked such surprise and concern among international nuclear inspectors that they are planning to confront Tehran about it this week.

Inspectors had long suspected that Iran had been working on the P-2 centrifuge design -- bought on the black market from the renegade Pakistani nuclear engineer Abdul Qadeer Khan -- separately from the activity at its main nuclear facility at Natanz.

European diplomats said a delegation of Iranian officials is due to arrive on Tuesday in Vienna, where the IAEA will press them to address the new enrichment claim, as well as other questions about Iran's programme, including a crude bomb design found in the country, The Times said.

If Iran moved beyond research and actually began running the machines, it could force American intelligence agencies to revise their estimates of how long it would take for Iran to build an atom bomb -- an event they now put somewhere between 2010 and 2015, according to the report.

In Israel -- which is widely believed to already have a nuclear arsenal -- the head of the ultra-nationalist Yisrael Beitenu party said the Jewish state may have to take its own pre-emptive action.

Avigdor Lieberman said Iran's nuclear programme "represents an existential threat for Israel which will oblige us to take unilateral action if the international community does nothing to stop it".

"The only difference between the aspirations of the madmen of the current regime in Tehran and Hitler is that their (Iranian) threat is more concrete," he added on Israeli radio.