Iran's President Mahmoud Ahmadinejad sparked outrage in the United States on Thursday when he told a UN meeting that most people believe the US government staged the September 11 attacks.

He sparked a walkout in the UN General Assembly shortly after US President Barack Obama had offered a negotiated settlement with Iran on its nuclear drive.

The US delegation led an enraged Western walkout after Ahmadinejad's comments on the al-Qaeda attack on the World Trade Centre, which was just six kilometres from the UN headquarters. European Union delegations quickly followed.

The Iranian president said there was a theory that "some segments within the US government orchestrated the attack to reverse the declining American economy and its grips on the Middle East, in order also to save the Zionist regime.

"The majority of the American people as well as other nations and politicians agree with this view," he declared to the astonished chamber.