WASHINGTON, Oct 5: The debate on possible nuclear threats to America entered a new phase on Tuesday when US officials said a top Al Qaeda leader had tried to obtain radioactive material for smuggling a so-called dirty bomb into the United States.

A dirty bomb is a conventional explosive laced with radioactive material and can have devastating effects if used in a densely populated area, particularly a large city.

Although the nuclear controversy always had a simmering presence in the United States, it occupied the central stage last week when in the first of the three presidential debates both President George W. Bush and his challenger John Kerry said they regarded nuclear proliferation as the most serious threat to US interests in the near future.

Both said they feared that terrorists might acquire nuclear technology from unsecured sources across the world and use it to threat US security. On Tuesday, US officials said that a top Al Qaeda leader, Adnan G. El Shukrijumah, was working on a scheme to obtain radioactive materials for a so-called 'dirty bomb' that could be smuggled into the United States.

Shukrijumah has attempted unsuccessfully to enter the United States using phony passports, US authorities said, adding that he had lived in South Florida before and worshipped at the same mosque as Jose Padilla, who is being held as an enemy combatant in a plot to detonate a 'dirty bomb'.

FBI agents had arrested Padilla, a Muslim convert also known as Abdullah al Muhajir, in May 2002 at Chicago's O'Hare International Airport after a flight from Pakistan. He was allegedly carrying $10,000 in from his Al Qaeda handlers.

Recent reports in US newspapers said Shukrijumah was observed last year during a trip to Canada, where authorities suspect he posed as a student at McMaster University in Hamilton, Ontario. An FBI informant told US authorities the Al Qaeda leader was seeking material to build a dirty bomb.

McMaster University has a five-megawatt research reactor, whose uranium-based fuel rods come from the United States. Canadian officials have denied any security breach of the McMaster facility.

The US State Department has offered a $5 million reward for Shukrijumah, who is also sought for questioning by the FBI in connection with terrorist threats against the United States. He was named in a March 2003 material-witness arrest warrant by prosecutors in Northern Virginia, where US officials said he was planning terrorist attacks.