PARIS, Jan. 16 - A Swiss laboratory announced today that it had found traces of a uranium isotope that suggest radioactive contamination in American-made munitions that were collected on the battlefields of Kosovo.

The lab, the Swiss Federal Institute of Technology in Spiez, said that the quantities found of uranium 236 were minute and that it was checking for other substances in the spent bullets. They were retrieved by a United Nations mission that was checking the effects of depleted uranium weapons.

Four other European labs are analyzing samples from Kosovo. Their joint findings of toxic materials found in soil, water and spent shells are to be published in March. It is not clear why the Swiss lab announced part of its results today.

The lab acted as a furor in Europe over sicknesses among NATO troops who are returning from Kosovo is shifting focus. Scientists and nuclear experts in Europe have said there are indications that some depleted uranium used in antitank rounds was "dirty," or contaminated.

The Swiss finding of uranium 236 is certain to increase anxiety in the debate over why 15 European troops recently died of leukemia and others have unexplained illnesses.

The announcement coincides with the publication of a book in France next week, "Depleted Uranium, Invisible War," that has researched the contamination of the depleted uranium and its consequences and is causing a stir in Paris.

NATO has tried to calm the uproar, saying depleted uranium munitions cannot cause serious health problems after impact. NATO quickly created a medical committee that repeated today that there was no recognizable "Balkans Syndrome."

Scientists and nuclear experts in Europe and the United States are at odds over the dangers.They also disagree on whether the presence of uranium 236 makes a crucial difference, even though it suggests that other contaminants may be present.

Everyone seems to agree that uranium 236 does not occur in natural uranium ore, nor is it meant to be found in depleted uranium, which is was stripped of the elements suitable for use in reactors and bombs.

"U 236 is created in a nuclear reactor," said a French nuclear physicist, Monique Sené. "It comes from nuclear fuel and, most likely, from recycling nuclear waste. There is no other known source."

An American physicist, Dr. Steve Fetter, said the presence of uranium 236 in munitions with depleted uranium was known but said it would not cause a health problem, because its alpha radiation does not allow it to wreak much damage. He said the uranium did not penetrate into the bone and marrow, where leukemia originates.

But Jean-François Lacronique, director of the National Radiation Protection Agency in France, a watchdog agency, said in an interview the finding of uranium 236 was a cause for concern because "it is 10 times more radioactive than depleted uranium, and it acts very quickly." When uranium-tipped munitions explode in a flame, Mr. Lacronique said, the high temperatures can turn uranium into tiny droplets or dust particles that can enter the body, where they can remain radioactive for 200 days.

He also said a report published in The European Journal of Nuclear Medicine stated that uranium 236 had been found in the urine and bone tissue of some Persian Gulf war veterans.

The presence of uranium 236 changes the scope of the health problems, Mr. Lacronique said. "To get cancer from depleted uranium, you have to be exposed for a long time to very large amounts. But U 236 changes the equation, because it comes from burnt nuclear fuel that was recycled. We now have the duty to find out if other contaminants from burnt fuel are present like plutonium or americium, which are much more harmful."