While there is no proof Usama bin Laden has nuclear weapons, a wide range of international analysts say he has been trying to acquire them for years and may have succeeded, or be close.

Even experts who think bin Laden's Al Qaeda network does not have an atomic bomb say it's best to assume it does and prepare for its possible use.

The scale and sophistication of the Sept. 11 terror attacks, in which bin Laden is the main suspect, mean nothing can ever again be ruled out, no matter how nightmarish, they say.

"The Sept. 11 attacks certainly take us a lot closer to a nuclear possibility," said Paul Wilkinson, an expert on terrorism at St. Andrews University in Scotland.

Fanatical terrorists "might resort to this kind of mass destruction weapon and we have to take that seriously," he said.

U.S. Army Gen. Tommy Franks, commander of U.S. military operations against the Taliban and Al Qaeda, said Tuesday that U.S. officials had identified more than 40 sites in Afghanistan where bin Laden's network may have been researching nuclear, chemical or biological weapons.

He said the sites were still being tested for evidence that any weapon of mass destruction had actually been produced.

Many experts fear bin Laden has a "dirty bomb" or could quickly construct one. Not capable of producing a nuclear explosion, a dirty bomb would use conventional explosives to spread radioactive material over a wide area and make it uninhabitable. It could be made from easily acquired low-grade nuclear material, such as isotopes for medicine and industry.

"The problems in finding materials for a dirty bomb practically do not exist," said Dmitry Kovchegin of the Center for Policy Studies in Moscow.

Bin Laden has boasted of having weapons of mass destruction. The U.S. and other Western governments say there is no evidence he has a nuclear weapon, but officials acknowledge it can't be ruled out.

"Inevitably, it will happen eventually," said Dr. Frank Barnaby, a nuclear physicist who specializes in nuclear terrorism studies at the Oxford Research Group, a private think tank in Oxford, England.

Analysts who doubt bin Laden has a nuclear bomb don't think Al Qaeda has the skill to develop such weapons. Plans for an atomic bomb found in Kabul, reported recently by the British press, appear to have been an old spoof from a humor journal that Al Qaeda may have mistaken for genuine diagrams.

But even the skeptical experts won't rule out the possibility bin Laden has the bomb.

"Making a bomb and getting it somewhere is a low likelihood scenario, but the consequences if they did are extremely high, so that pushes the risk level up. So I would say the risk level is medium," said Clive Williams, a terrorism expert at Australian National University.

Experts have worried for years that acquiring or building a nuclear weapon of some kind is a much greater possibility since the collapse of the Soviet Union.

Criminals have been caught smuggling nuclear material out of Russia. There are numerous, unverified reports of nuclear weapons being sold or misplaced. Russian officials admit security at their nuclear facilities is often poor. There is concern penniless nuclear scientists might be hired by outsiders to develop weapons.

"Undoubtedly the disintegration of such a huge state as the U.S.S.R. created temptations and it would seem strange if nobody took advantage of them. Such organizations as Al Qaeda have enough money and organizing skills to do it," said Vladimir Lukin, a vice speaker of the Duma, the lower chamber of the Russian parliament.

Nuclear weapons in the arsenals of the United States, Russia and the other nuclear powers can only be operated with codes. Even if bin Laden has an ex-Soviet weapon, he is unlikely to have the code to detonate it, experts say.

Analysts are divided over the chances of terrorists building a nuclear bomb. Some say it is easier than generally realized. Others counter that Iraq apparently failed to build a bomb despite a $10 billion effort lasting years.

Barnaby said an atomic bomb can be built fairly easily using highly enriched uranium. Only Pakistan uses this material to build atomic bombs, which is worrisome, he said, because of known links between bin Laden and some former Pakistani nuclear scientists.

But if bin Laden had a nuclear weapon, wouldn't he have used it?

Analysts aren't so sure, saying terrorists traditionally build up attacks to heighten terror, so a nuclear attack might be saved for a final blow. The collapse of Al Qaeda in Afghanistan could increase the chances, they say.

"They would want to keep things up their sleeves. Terrorists need to escalate attacks. They have to notch it up all the time," said Barnaby.

"The next natural move would be a nuclear terrorist act."