IENNA, Nov. 10 ?In the last year, there have been dozens of violations of nuclear security rules in Russia and at least one loss of fissile material; Taliban emissaries have tried to recruit Russian scientists, and terrorists have tried to stake out a Russian nuclear storage site at least twice, say senior officials of the International Atomic Energy Agency and Western governments.

The officials detailed the incidents, citing conversations with Russian officials and verified news reports. Despite significant improvements in Russian nuclear security in the 1990's ?some of it with American money and advice ?up to half of ex-Soviet civilian and military nuclear stockpiles with weapons-grade material are not well protected.

Officials of the Vienna-based International Atomic Energy Agency, the United Nations body for monitoring nuclear programs, are deeply skeptical of Osama bin Laden's claim, in an interview published in Pakistan on Friday, that he possesses nuclear weapons.

On the other hand, given the vulnerability of material in the former Soviet Union, the increasing professionalism of nuclear smuggling and the relative ease of fabricating a primitive weapon, they cannot rule it out.

In the Kazakh port of Aktau on the Caspian shore, one ton of plutonium and two tons of highly enriched uranium sit near a now closed breeder reactor.

Ukraine, with 17 nuclear reactors and one research reactor, is considered a country of "serious concern" by officials because of its climate of government corruption and crime. Enough highly enriched uranium to make a bomb remains at a research reactor just outside Belgrade throughout the 1999 Kosovo war.

Just last week, Turkey announced it had broken up a gang of smugglers who tried to sell 2.2 pounds of what appeared to be highly enriched uranium for $750,000 to undercover police officers, material they said they had bought several months ago from a Russian of Azeri origin.

Officials are increasingly concerned that terrorists willing to die could create a "dirty bomb," wrapping more easily stolen radioactive materials used in medicine and industry around a conventional explosive, like dynamite, to try to make a significant area of a city uninhabitable for many years.

Russian officials say their fissile nuclear material is under strict and improving controls. But only 10 days ago, in a discussion with officials at the United Nations agency here, Yuri G. Volodin, chief of safeguards for the Russian nuclear regulatory agency, revealed that in the last year, there were dozens of violations of Russia's regulations for securing and accounting for nuclear material.

Mr. Volodin noted one loss of nuclear material, which he called of the "highest consequence." He said he could not be more specific about the type of material or the size of the loss.

Last month, Col.-Gen. Igor Volynkin, head of nuclear security for Russia's military, said that twice this year Russian forces discovered stakeouts by terrorists of a secret nuclear arms storage facility, although he did not say where.

Also last month, an official of the Russian Security Council, Raisa Vdovichenko, told Russian journalists that emissaries of the Taliban had asked an employee of "an institution related to nuclear technologies to go to their country to work there in this field."

There is continuing evidence of efforts to traffic in nuclear material that give many officials deep concern.

In April 2000, the police in Georgia seized, in Batumi, several hundred fast-reactor fuel pellets, containing 920 grams ?nearly a kilogram ?of highly enriched uranium; in September, at Tbilisi airport, the police confiscated half a gram of plutonium.

The Russians say they thwarted an effort, at the very end of 1998, by an organized gang to steal 18.5 kilograms ?more than 40 pounds ?of highly enriched uranium from a military weapons facility near Chelyabinsk in the Urals.

Still, senior officials here and in Washington do not believe that Mr. bin Laden or even any state interested in a shortcut to a bomb ?from Syria and Iran to Iraq and Libya ?has been able to obtain the roughly 25 kilograms (55 pounds) of highly enriched uranium required to make a simple bomb, or the roughly 8 kilograms (17.6 pounds) of plutonium, a much more difficult material with which to work.

But they also admit that they cannot possibly know for sure.

The atomic energy agency has built a database of incidents of nuclear trafficking since 1993 ?only counting incidents confirmed by the states involved. Of the 175 cases of trafficking in nuclear material and 201 cases of trafficking in medical and industrial radioactive materials, only some 18 cases involved even small amounts of the fissionable material needed for a nuclear bomb ?plutonium or highly enriched uranium (enriched by 20 percent or more).

Altogether in all these cases, agency officials say, there have been seizures of about 400 grams (nearly one pound) of plutonium and an additional 12 kilograms (26.4 pounds) of uranium at varying levels of enrichment, equivalent to only some 6 kilograms of uranium 235.

The most serious cases, involving large amounts of material, took place in 1993 and 1994, when Russian, German and Czech police officers made large seizures of very highly enriched nuclear material manufactured in the former Soviet Union, usually at nuclear-fuel fabrication plants.

In March 1993, in St. Petersburg, nearly three kilograms (6.6 pounds) of 90 percent enriched uranium-238 were seized; in August 1994, in Munich, the police seized about 360 grams of Russian-made plutonium; in December 1994, 2.7 kilograms (just over 5 pounds) of 80 percent enriched uranium-235 were seized, part of a shipment that showed up in smaller amounts in other places ?and which officials hope was not part of an even larger shipment, apparently stolen from the Russian nuclear research center in Obninsk, about an hour's drive southwest of Moscow.

For context, officials point out, Iraqi leader Saddam Hussein had made only 1.5 kilograms (3.3 pounds) of bomb-capable uranium before the gulf war broke out.

But in fact the atomic energy agency's database is only a guide, and perhaps not even a good one. "Are we seeing half the iceberg or only the tip?" said one official, noting that the police consider seizures of drugs, a commodity far easier to secure, to represent only some 10 to 20 percent of what is shipped. Nor does the agency, devoted to civilian nuclear energy, know much about the military programs of states with nuclear weapons.

Friedrich Steinh?sler, a physics professor at Stanford University and co-director of a Stanford center on the physical protection of nuclear materials, said, "It's clear that we're seeing a typical move toward professionalism in this smuggling business, with increasingly fewer incidents of significance, but of greater significance, as professionals are probing the market."

He noted that traffickers increasingly are going south, over traditional smuggling routes through Turkey, the Caucasus and especially central Asia, closer to Afghanistan, where borders are extremely long and lax.

Matthew Bunn, assistant director of the science, technology and public policy program at Harvard University's Kennedy School, was a Clinton White House adviser. The main source of loose nuclear material remains the former Soviet Union, he says, with some 600 tons of weapons- grade nuclear material stored there outside of warheads.

The key question, he says, is to improve the security around military and especially civilian nuclear installations. In as many as half, he said, there are no automatic detectors that sound an alarm if material is smuggled out, and no security cameras where material is stored.

"For all the work we've done with Russia, after seven years, we still have most of the job to do," Mr. Bunn said. "This is a serious threat, and we know how to fix it," he said, urging that President Bush agree with Russia at the this week's summit meeting to account for and secure all nuclear material.

Some safeguards put in place by the Americans in the former Soviet Union no longer function, agency officials said ?spare parts are expensive and available only from the United States, and sometimes guards do not bother to use the equipment.

The Vienna agency is also looking for a 10 percent increase in its own budget of some $320 million, said Graham Andrew, the special assistant for Scientific and Technical Affairs, to upgrade security standards around the world. He and other officials regard a terrorist nuclear bomb to be "highly unlikely."

But the likelihood of terrorists compiling the radioactive materials necessary to make a dirty bomb with immense economic and psychological impact is much higher, the officials say.

The dirty bomb is an almost ideal instrument of terror, Mr. Bunn said. It would not kill many people, but it would terrify, and make a large area unsafe to work or live in, possibly for decades or longer.

One official said: "Imagine a dirty bomb on the Washington mall. Do you abandon the White House?"