A Russian man who tried to sell a small piece of weapons-grade uranium has been arrested in Georgia, officials say.
The man was detained in the Georgian capital Tbilisi last summer in a sting operation involving US agents, the Georgian interior minister said.
Vano Merabishvili said he was giving details now because Russia had failed to co-operate over the case.
The Russian was carrying 100g (3.5oz) of uranium, but had offered more. A US test confirmed it was highly enriched.
Experts at the US Department of Energy examined the sample and concluded it was powerful enough to fuel part of a nuclear weapon.
The man was able to transport it in a plastic bag in his pocket, the Associated Press reported, because uranium has a low level of radioactive emission.
He has been identified as Oleg Khintsagov, from the southern Russian region of North Ossetia.
The BBC's Matthew Collin, in Georgia, says the case raises new concerns about militants gaining access to nuclear material, particularly in conflict zones in the former Soviet Union where the rule of law is weak and corruption is widespread.
According to Mr Merabishvili, the Russian said the uranium was just a sample of a much larger amount he had available to sell.
But these claims were never substantiated, US and Georgian officials indicated.
Mr Merabishvili said Russia had not yet responded to an offer by Georgia to hand over information about the case.
Georgian efforts to trace the nuclear material since the arrest and confirm whether the man did have access to larger quantities have foundered from a lack of cooperation from Russia, he said.
"We were ready to provide all the information, but unfortunately no-one arrived from Russia, not even to interview this person," Mr Merabishvili said.
"It is surprising because it is in Russian interests to secure these materials. There are terrorist organizations in Russia who would pay huge amounts of money for this."
Relations between Russia and Georgia have been tense in recent months, following a row about alleged spying by Russians and Moscow's expulsion of Georgian illegal workers.
A spokeswoman for the International Atomic Energy Agency, Melissa Fleming, said the arrest was of vital importance.
"Given the serious consequences of the detonation of an improvised nuclear explosive device, even small number of incidents involving HEU [highly enriched uranium] or plutonium are of very high concern," she said.
Georgia said it became aware of the smuggling plot while investigating crime in the breakaway republics of South Ossetia and Abkhazia.