WASHINGTON - Republic of Georgia authorities, aided by the CIA, set up a sting operation last summer that led to the arrest of a Russian man who tried to sell a small amount of nuclear-bomb-grade uranium in a plastic bag in his jacket pocket, U.S. and Georgian officials said.

The operation, which neither government has publicized, represents one of the most serious cases of smuggling of nuclear material in recent years, according to analysts and officials.

The arrest underscored concerns about the possibility of terrorists acquiring nuclear-bomb-making material on the black market, although there was no suggestion that this particular case was terrorist-related.

Details of the investigation were provided to the Associated Press by U.S. officials and Georgian Interior Minister Vano Merabishvili.

Authorities say they do not know how the man acquired the nuclear material or if his claims of access to much larger quantities are true. He and three Georgian accomplices are in Georgian custody and not cooperating with investigators.

Georgian attempts to trace the nuclear material since the arrest and confirm whether the man indeed had access to larger quantities have foundered from a lack of cooperation from Russia.

Merabishvili said he was revealing the story out of frustration with Russia's response and the need to illustrate the dangers of a breakdown in security cooperation in the region.

A message left with the press office of the Russian Embassy was not returned. A duty officer at the Russian Foreign Ministry said there was no one authorized to comment last night.

According to Merabishvili, a Georgian undercover agent posing as a rich foreign buyer made contact with the Russian seller in North Ossetia, which is part of Russia. After the Russian offered to sell the sample, the agent insisted the Russian come to Tbilisi, the Georgian capital.

At a meeting in Tbilisi, the man pulled out from his pocket a plastic bag containing the material.

The man was arrested and sentenced to eight to 10 years in prison on smuggling charges. His accomplices were sentenced on lesser charges.

Russian authorities took a sample of the material but failed to offer any assistance despite requests for help from the Georgians, Merabishvili said.

The Georgians asked for U.S. assistance. Agents from the FBI and the Energy Department took the material back to the United States, where it was tested by the Energy Department's National Nuclear Security Administration.

"The material was analyzed by agency nuclear experts," said Bryan Wilkes, a spokesman for the the agency, "and confirmed to be highly enriched uranium."