MOSCOW - Russia responded with silence yesterday after Georgia revealed a foiled effort by a Russian to sell weapons-grade uranium, an episode that appeared to cast doubt on Moscow's ability to halt the black-market trade in nuclear materials.

The origin of 3.5 ounces of highly enriched uranium seized early last year in the former Soviet republic remains unclear, and some experts accused Georgia of trying to embarrass Russia at a time of strained U.S.-Russian ties.

The Russian government said nothing publicly about the inquiry. An unidentified official at the nuclear agency Rosatom, quoted by the Interfax news agency, denied Georgian accusations that Russia was not cooperating with an investigation.

U.S. and Georgian officials told the Associated Press that Georgian authorities, aided by the CIA, set up a sting that led to the arrest last year of a Russian citizen who tried to sell a small amount of uranium enriched to about 90 percent U-235, suitable for use in an atomic bomb.

Georgian officials said attempts to find the source of the nuclear material, and to investigate the man's claim that he could get more, failed because Russia did not cooperate.

The Rosatom official was quoted as saying that Georgian authorities had given Russia too small a sample to determine its origin and had refused to provide other information.

A Georgian Interior Ministry official, Shota Utiashvili, identified the detained man as Oleg Khinsagov, a resident of Vladikavkaz in North Ossetia, a region of Russia that borders Georgia.

Utiashvili said Georgian authorities had thwarted an earlier smuggling attempt in 2003 also involving a small amount of highly enriched uranium, but gave no further details.

Alexander Pikayev, a Moscow-based defense analyst who is cochairman of the Committee of Scientists for Global Security, said there had been thefts of nuclear material from Russian facilities in the past.

"This incident shows that all is not well," he said.

According to the Nuclear Threat Initiative, a U.S. nongovernmental organization devoted to nonproliferation issues, Russia now has from 735 to 1,365 metric tons of weapons-grade-equivalent highly enriched uranium and from 106 to 156 tons of military-use plutonium.

In a 2006 report, the International Atomic Energy Agency said there were 16 confirmed incidents of trafficking in highly enriched uranium or plutonium globally from 1993 to 2005. In seven cases, the nuclear material was thought to originate in Russia or a former Soviet state.