Two Armenian men have pleaded guilty during a secret trial to smuggling highly enriched uranium into Georgia, officials said Monday, highlighting concerns over loose nuclear materials in the ex-USSR.

Sumbat Tonoian and Hrant Ohanian were arrested in a sting operation in March after they smuggled the 18 grams (0.6 ounces) of uranium from Armenia into Georgia and tried to sell it to an undercover agent, Interior Ministry spokesman Shota Utiashvili told AFP.

He said they tried to sell the material for 1.5 million dollars to an agent they believed represented Islamic radicals.

Utiashvili called the operation "a big success for our nuclear smuggling unit", after Georgia in recent years received nearly 50 million dollars in aid from Washington to help it combat trafficking in nuclear materials.

Media reports said the two men had smuggled the uranium on a train from the Armenian capital Yerevan to Tbilisi in a cigarette box lined with lead to fool radiation sensors at the border.

The reports said tests had confirmed that though a small amount, the uranium was nearly 90 percent enriched and potentially useable in a nuclear warhead.

Reports said the two men were attempting to sell the uranium as a sample and had said they were able to obtain more. Tonoian was described as a failed businessman and Ohanian as a retired physicist.

Georgian President Mikheil Saakashvili first informed world leaders of the case at a nuclear summit in Washington in April, but Utiashvili said full details could not be released until the men had pleaded guilty.

The two are facing at least 10 years in prison each, he said, adding that the trial was held in secret in Georgia to protect the identity of undercover agents.

The case has highlighted concerns that unsecured nuclear materials around the former Soviet Union could be smuggled through the region's porous borders and used to build nuclear weapons.

Lawrence Sheets, Caucasus project director for the Brussels-based International Crisis Group, said it was "extremely frightening" that it was unclear how much nuclear material was unaccounted for in the region.

"Nuclear proliferation experts will tell you that we don't know how much highly enriched uranium actually existed in Soviet stockpiles and we don't know how much leaked out in the 1990s," he said.

It was the third case of smuggling of nuclear materials to be uncovered in Georgia, an ex-Soviet republic on Russia's southern border closely allied to the United States.

In a joint US-Georgia sting operation in 2006, officials arrested a Russian citizen trying to sell 100 grams (3.5 ounces) of highly enriched uranium to a Georgian officer posing as a buyer from a radical Islamic group.

Three years earlier, Georgian border guards intercepted an Armenian national carrying highly enriched uranium as he tried to cross from Georgia into Armenia.

Sheets said that despite the arrests in Georgia, it was unfair to label the country as the worst offender in the region because it was one of the few actually stopping and revealing cases of nuclear smuggling.

"It looks like a lot of material is coming through Georgia but in my opinion that's a sign of the fact that there's a very competent effort to combat nuclear smuggling in Georgia," he said.