A leading Pakistani nuclear scientist, barred by his government from talking to reporters, has made it known through his son that Usama bin Laden approached him before the Sept. 11 attacks for help in making nuclear weapons.

The Al Qaeda leader was rebuffed, the son, Azim Mahmood, said in a recent interview with The Associated Press.

"Basically Usama asked my father, 'How can a nuclear bomb be made and can you help us make one?'" he said. "My father said, 'No, and secondly you must understand it is not child's play for you to build a nuclear bomb.'"

The scientist, Sultan Bashiruddin Mahmood, is under a gag order from Pakistani intelligence officials, but his conversations with bin Laden in meetings in 2000 and as late as July 2001 were reconstructed for the Associated Press by his son.

The conversations as described by Azim Mahmood clearly show bin Laden was interested in developing nuclear weapons. They don't, however, shed any light on whether the terrorist mastermind had taken even the first steps on that complex technological challenge.

The U.S. Embassy declined to discuss Mahmood's story. American officials in Washington also would not comment.

There has been previous evidence of Al Qaeda's interest in nuclear weapons.

Computers found by journalists and U.S. troops at a variety of facilities in Afghanistan indicated Al Qaeda had sought to obtain and develop nuclear and other potent weapons. An AP reporter saw anthrax and other chemical concoctions at an Al Qaeda laboratory outside Kabul.

During a New York trial two years ago stemming from bombings at two U.S. embassies in Africa, a former bin Laden aide testified he was ordered in 1993 to try to buy uranium on the black market for an effort to develop a nuclear weapon. Jamal Ahmed Al-Fadl said Al Qaeda was prepared to spend $1.5 million, but he didn't know if a purchase was ever made.

In addition, U.S. officials have said captured Al Qaeda lieutenant Abu Zubaydah told American interrogators the terrorist network was working on a "dirty bomb," a conventional bomb that would scatter radioactive material. Such a radiological weapon would be far less deadly and damaging than a nuclear explosion.

Authorities also have said that Jose Padilla, the former Chicago gang member charged with plotting with Al Qaeda, attended two meetings in Karachi, Pakistan, at which senior Al Qaeda operatives discussed the possible use of a "dirty bomb."

A United Nations report issued by experts monitoring Al Qaeda movements warned that Al Qaeda has the potential to obtain nuclear material and build "some kind of dirty bomb."

"Our concern is you can actually get the stuff," said Michael Chandler, the British expert who heads the monitoring group.

The conversations related by Azim Mahmood confirm bin Laden's nuclear ambitions. But they also offer a glimpse at the nexus of science and conservative Islam at a high level in Pakistan, one of the world's newest nuclear powers along with neighboring India, whose own leaders follow a Hindu fundamentalist philosophy.

The elder Mahmood, who has been questioned by the FBI and is under close Pakistani surveillance, is a deeply conservative Muslim who espouses the same puritanical brand of Islam as Afghanistan's former Taliban rulers.

Enraged over Pakistan's plans in 1998 to sign the Comprehensive Test Ban Treaty, he resigned from the Pakistan Atomic Energy Commission and devoted his time to his charity, the Holy Quran Research Foundation.

Last December, President Bush labeled the charity a terrorist group and Mahmood a terrorist. His assets and those of his charity were frozen.

"Even my father's pension is blocked. At the moment he has nothing," said Azim Mahmood, a physician in his 30s who also adheres to a strict Islam.

For years, Pakistani peace activists and liberal academics have fretted about Islamic hard-liners in Pakistan's nuclear organization.

"We have always expressed our fear that a large number of people in the nuclear establishment would be ideologically motivated to share Pakistan's nuclear weapons technology," said Dr. A.H. Nayyar, a nuclear physicist and research fellow at the Sustainable Development Policy Institute, an independent Pakistani group.

Azim Mahmood said his father met with bin Laden in Afghanistan several times, "and definitely this question of building a nuclear bomb came up."

The father was detained in November 2001, questioned and freed in February, but has to carry a mobile phone at all times so Pakistani intelligence can track his movements, the son said.

He said his father's American interrogators were particularly intrigued by one of his books, "Doomsday and Life After Death," and wanted to know whether it meant he had some kind of inside knowledge of what Al Qaeda was planning.

Mahmood first met bin Laden in 2000 while visiting Afghanistan to build a school, the son said. He wanted to help the Taliban, because he was angry at the international criticism of the regime's brand of Islam, the son recalled.

"My father shared the Taliban thinking. He liked their system of government. He wanted to help them."

When bin Laden learned a nuclear scientist was in Kabul, he sent an Al Qaeda operative, Abu Bilal, to the Pakistani's hotel to arrange a meeting, the son said.

"My father went to meet him and he said, 'Why don't you come and help us build these things?'" Azim Mahmood said, adding that the two men met several times in the Afghan capital and the discussion invariably returned to nuclear weapons.

The Al Qaeda leader wanted a nuclear device, Azim Mahmood said. "Al Qaeda also wanted a person who could train their people, and who could get them enriched material for their weapons."

Experts say, however, that making a nuclear bomb requires a cadre of highly trained, experienced scientists and technicians.

In a separate interview, a former senior Taliban official said bin Laden was trying to obtain nuclear materials, but he could not say whether the Al Qaeda leader succeeded.

Mullah Mohammed Khaksar, who renounced the Taliban last year but had made contact with U.S. officials in 1999, said he knew of several mysterious shipments that entered Afghanistan and were stored at a warehouse in Kandahar, a Taliban stronghold. One was a balloon-like container covered in aluminum and others were capsules the length of a man's hand, he said.

Azim Mahmood said his father was uncertain what nuclear material, if any, Al Qaeda possessed.

"At one meeting they brought a box, a thing that someone had sold to them for a huge amount of money, but my father laughed and said it was nothing," he said.