Muhammad al-Baradai, head of the International Atomic Energy Agency (IAEA), said in an interview in Vienna with Norway's commercial TV2 channel on Saturday, that al-Qaida had been actively looking to acquire a nuclear weapon and other weapons of mass destruction.
TV2's website quoted al-Baradai as saying that proof had been found in Afghanistan, where US-led troops toppled the Taliban government in 2001.
"I would be surprised if they did not try to acquire nuclear weapons and other weapons of mass destruction. That would be the most horrible scenario because these extremist groups - if they have the weapon, they will use it," al-Baradai said.
"They were actively looking into acquiring a nuclear weapon and other weapons of mass destruction."
He said there was a race against time to stop the spread of nuclear weapons and to plug gaps in the security of atomic weapons and materials.
"The more nuclear weapons that exist, the more threat we are facing. And the more countries that have nuclear weapons, the more danger we are facing," he said.
"We can't afford one single lapse in the system of security of nuclear material or nuclear weapons."