Iran announced today that it was embarking on a plan to install 6,000 centrifuges to enrich uranium at its main nuclear plant, in defiance of United Nations sanctions.

President Mahmoud Ahmadinejad made his announcement at Natanz, in central Iran, as the country marked a "national day of nuclear technology", which falls on the day in April 2006 when uranium was successfully enriched for the first time.

According to the latest report by the UN's nuclear watchdog, Iran has already installed around 3,000 centrifuges at an underground enrichment facility in Natanz.

Nuclear experts believe between 20,000 and 30,000 centrifuges would be required to develop an actionable weapon, a goal which Mr Ahmadinejad denies, but the Iranian President hopes that developing 6,000 would send a powerful signal to the West that he is serious about developing a programme which he says is for the country's long-term energy needs.

The West fears Iran is pursuing a secret nuclear weapons agenda, and Tehran’s refusal to suspend the process has been punished with three sets of UN Security Council sanctions and separate US measures penalising its banking system and revolutionary guards.

Announcing an expansion, Mr Ahmadinejad said: "Today, the phase for installing 6,000 new centrifuges at the facility in Natanz has started."

After inspecting what he described as a "new generation" of centrifuges, Mr Ahmadinejad said he would be announcing more "good news" at a ceremony at 16.00GMT today at the headquarters of Iranian state broadcasting in Tehran alongside Gholam Reza Aghazadeh, the head of Iran’s atomic energy organisation.

"We have reached new achievements and in the ceremony I shall talk about this," Mr Ahmadinejad was quoted by the website as saying.

Tehran has insisted that it has no intention of making concessions over calls for it to freeze enrichment, leading to deadlock in the standoff with the international community. It insists that its nuclear programme is peaceful and aimed at generating energy for a growing population whose supply of fossil fuels will eventually run out.

However, the United States is concerned at the Iranian President's extreme rhetoric, including predictions that Israel is doomed, repeated denunciations of the West, and active training and financing of the Middle Eastern Islamist groups Hamas and Hezbollah.

Responding to today's announcement, the US said that it amounted to an "escalation" by Mr Ahmadinejad. President Bush has refused to rule out military action against Iran if it does not conform to international demands.

“Today’s announcement reflects the Iranian leadership’s continuing violation of international obligations and refusal to address international concerns,” Gregory Schulte, US representative to the Vienna-based International Atomic Energy Agency (IAEA), said.

“This approach has not brought Iran international respect or accolade but rather increasing censure and sanction. Negotiation, not escalation, provides the best path to international respect and regional security.”

Meanwhile the Iranian Fars news agency reported that a court had handed a former Iranian nuclear negotiator a two-year suspended jail sentence under charges of "harming national security".

Hossein Moussavian was a leading negotiator in the team that held talks with European Union countries during the presidency of reformist Mohammad Khatami until 2005.

He was briefly detained and then released on bail in 2007 on accusations of handing secrets about Iran’s controversial nuclear programme to the British embassy in Tehran. It is not known whether his current sentence was related to those accusations.

The AFP news agency quoted a source as saying only that he had been "sentenced to two years suspended in jail and a five year ban from holding state posts for harming the country’s security".

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