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Iranian President Mahmoud Ahmadinejad has said he believes - as an engineer - the World Trade Center towers could not have been brought down by aircraft.

Interviewed after his address to the UN General Assembly sparked a walkout, he told the AP news agency some kind of planned explosion must have occurred.

But he stopped short of saying the US staged the disaster 10 years ago.

He had been widely condemned for using his UN address to brand the US killing of Osama Bin Laden a 9/11 cover-up.

Diplomats from more than 30 countries, including the US and EU nations, left the hall as he attacked the West, denounced Israel and questioned the Holocaust.

The mass walkout has become almost a staple of the General Assembly's meetings. It came a day ahead of a Palestinian bid for UN membership.

Mr Ahmadinejad told AP it was not too late for Iran and the US to repair their relationship.

But he was fiercely critical of the operation against Osama Bin Laden, when, he said, the US "killed the main perpetrator [of 9/11] and threw his body into the sea".

"Would it not have been reasonable to bring to justice and openly bring to trial the main perpetrator of the incident in order to identify the elements behind the safe space provided for the invading aircraft to attack the twin World Trade Center towers?" he had told the assembly.

The Iranian president has previously called the 9/11 attack a "suspect event" and a "big fabrication" used to justify the US war on terrorism.

White House press secretary Jay Carney told reporters aboard Air Force One that he "found it rich" that Mr Ahmadinejad would criticise US policy, and accused Tehran of "vile mistreatment" of its own citizens.

Mr Ahmadinejad had also used Thursday's address to accuse Western nations of "weakening countries through military intervention and destroying their infrastructures, in order to plunder their resources by making them all the more dependent".

UK Prime Minister David Cameron hit back at Iran during his speech to the UN.

"They do everything they can to avoid the accountability of a free media," he said.

"They violently prevent demonstrations and, yes, they detain and torture those who argue for a better future."

Hundreds of people protested against the Iranian leader's appearance at the UN meeting in New York.

Across from the UN headquarters, there were huge banners, one of which read, "Down With the Islamic Republic of Iran", while children stamped on a poster of Mr Ahmadinejad.

The foreign ministry of Israel, which boycotted the speech, said in a statement: "Once again the Iranian president delivered a message of hostility to the family of nations, threatening world peace and security."

On Friday the UN Security Council is expected to examine a bid to admit the Palestinians as a partial or full member of the UN.

The US has vowed to veto the request, which is also firmly opposed by Israel.

US President Barack Obama has said there can be "no short-cut" to a negotiated peace deal in the Middle East.