Some of the former US hostages have said they recognise Mr Ahmadinejad as one of their captors.

But three Iranians involved in the action, as well as Mr Ahmadinejad's own staff, have denied that he took part.

Mohsen Mirdamadi, the hostage-takers' leader, told the BBC that the new president had not been there.

You could make him a blond and shave his whiskers, put him in a zoot suit and I'd still spot him Former US hostage Chuck Scott

Another top student leader, Abbas Abdi, also denied the allegations as did Hamid Reza Jalaiepour, a third hostage-taker.

The BBC's Frances Harrison in Tehran says the three former students are now reformists who oppose hardliners like Mr Ahmadinejad, and would have no reason to hide his involvement now.

Questions raised

Photographs have appeared on the internet showing a young bearded man leading a blindfolded American hostage - alleging that this was Mr Ahmadinejad a quarter of a century ago.

US hostages differ over whether the new president was their captor

But the man in the photograph appears much taller than Mr Ahmadinejad, and looks nothing like other pictures of him as a student which can be found on his website, Frances Harrison says.

White House spokesman Scott McClellan said the allegations should be taken seriously.

"I think the news reports and statements from several former American hostages raise many questions about his past," he said.

"We take them very seriously and we are looking into them to better understand the facts."

One former hostage said he was sure the man was Mr Ahmadinejad.

"This is the guy," retired army colonel Chuck Scott told AP news agency.

"There's no question about it. You could make him a blond and shave his whiskers, put him in a zoot suit and I'd still spot him."

Four other former hostages agreed with Mr Scott, but at least one disagrees.

A total of 52 US citizens were taken hostage after a group of young activists climbed over the embassy wall during the Islamic Revolution, which overthrew the Shah and brought Ayatollah Khomeini to power.