Iran says it is enriching uranium for a civilian power programme
Iran has begun installing 6,000 new centrifuges at its main nuclear plant in Natanz, President Mahmoud Ahmadinejad has said.
He also appeared to cast doubt on the official version of the 9/11 attacks and accused the US of using them as a "pretext" to launch invasions.
Iran is already thought to have some 3,000 centrifuges - which are used to enrich uranium - at its Natanz plant.
The US and Western allies condemned him but disagreed on how to respond.
Later, Mr Ahmadinejad claimed Iran had tested on Tuesday advanced new equipment - thought to be a reference to a possible new generation of centrifuges.
'Five times greater'
"The capacity of these new machines... is five times greater than the current machines," he told a televised event in Tehran.
He made the comments following a visit to Natanz to mark national nuclear day.
Iran has been enriching uranium at its underground facility in Natanz
Turning his attention to the 11 September 2001 attacks on the US, Mr Ahmadinejad said: "On the pretext of this incident a major military operation was launched and oppressed Afghanistan was attacked.
"Tens of thousands of people have been killed until now. Poor Iraq was attacked."
He also alleged the names of the people killed in the attacks on New York's World Trade Center had never been published.
And he added: "How is it possible that with the best radar systems and intelligence networks the planes could crash undetected into the towers?"
US Secretary of State Condoleezza Rice appealed to Tehran to 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.
French Foreign Minister Bernard Kouchner suggested sanctions on Iran could be "reinforced".
Last month, the UN Security Council approved a third round of sanctions against Iran.
But Russian Foreign Minister Sergei Lavrov instead suggested a new package of incentives to encourage Iran to halt uranium enrichment.