President Ahmadinejad was jeered when he addressed Columbia

"We're not taking it too seriously," said a White House spokeswoman.

She said Mr Bush might have considered the invitation if Iran allowed freedom of expression, did not have nuclear ambitions, and did not threaten Israel.

Columbia University's head last week introduced Iran's Mahmoud Ahmadinejad as a "petty and cruel dictator".

The head of Ferdowsi University in Iran's second city of Mashhad - one of the country's oldest universities - said Mr Bush could answer students' questions about the Holocaust, terrorism and human rights.

"This is what President Ahmadinejad did, despite the lack of respect shown towards him," said Ferdowsi University president Alireza Afshour, according to the government daily newspaper Iran.

Columbia University president Lee Bollinger last week condemned Mr Ahmadinejad for denying the Holocaust took place, saying he was either "brazenly provocative or astonishingly uneducated".

Mr Ahmadinejad's assertion that there were no gays in Iran also drew jeers from the audience.

A White House spokeswoman responded to the invitation on behalf of Mr Bush: "If Iran was a free and democratic society that allowed its people freedom of expression, and wasn't pursuing nuclear weapons, and wasn't advocating to destroy the country of Israel, the president might consider that invitation."

Protests at home

Earlier this week, on his return from New York, President Ahmadinejad was due to speak at Tehran University.

Students there wrote him a letter asking about the academic freedoms he had described to his New York audience. They complained about arrests of students and staff members and what they said were the appalling punishments handed out to critics of the president.

They asked to be allowed to meet Mr Ahmadinejad and when they received no reply, they threatened to stage a protest outside the hall, says John Leyne, a BBC correspondent in Tehran.

Shortly afterwards President Ahmadinejad cancelled his visit, though his office said it was because of the current religious festival being celebrated in Iran, our correspondent says.