U.S. forces have uncovered evidence that Usama bin Laden may have begun developing weapons of mass destruction, Defense Secretary Donald H. Rumsfeld said Sunday.

Some sites in Afghanistan that may have been used to create chemical, radiological or biological weapons have been bombed by the U.S., Rumsfeld said. Other sites have been left alone, and others likely have not been found, he added.

Once there are indications that such weapons are being produced at a specific location, "are you best taking it out or are you best learning more about it?" Rumsfeld asked rhetorically on Fox News Sunday.

The New York Times reported Sunday that the United States had identified three possible weapons-building sites in Afghanistan used by bin Laden's Al Qaeda network, and had avoided bombing them.

In a recent interview with a Pakistani journalist, bin Laden claimed to have weapons of mass destruction, but said he would not use them unless the United States used such weapons first.

Rumsfeld said he is worried bin Laden's boast of having weapons that could kill hundreds or several thousands at time may be true. He said Al Qaeda is known to have links to terrorism-sponsoring countries that can make chemical, biological and radiological weapons.

From there's it's not a far step to the possibility that perhaps the world's most dangerous has a hold of some of the world's most dangerous weapons.

"It does not take a leap of intelligence to know that if they have a relationship ... then [bin Laden] either has them, will have them or wants to have them," Rumsfeld said. "I know a lot about it from intelligence. On the other hand, you have to know a lot more to know precisely whether you have these capabilities that are readily usable."

Radiological weapons are made with nuclear material, combined with conventional explosives, and spread radiation without creating a nuclear blast.

Other leaders were skeptical of bin Laden's claims.

Condoleezza Rice, President Bush's national security adviser, said there is "no credible evidence" bin Laden had acquired nuclear weapons. Secretary of State Colin Powell said he thought it unlikely the Saudi exile had such weapons.

Rice added that America is doing everything possible to make sure Al Qaeda does not get chemical or biological weapons.

"We believe that we have a very, very aggressive program under way to make certain that he does not acquire them," Rice said on ABC's This Week.

"We have every intelligence operation practically in the world on the problem of Al Qaeda and the Taliban and their weapons of mass destruction at this point."

The Associated Press contributed to this report