Two Armenian men have pleaded guilty during a secret trial in Tbilisi, the Georgian capital, to smuggling highly enriched uranium into Georgia, officials have said.

Sumbat Tonoian, a businessman and Hrant Ohanian, a retired physicist, were arrested in a sting operation in March after they smuggled 18 grammes of weapons-grade material in a lead-lined cigarette packet on a train from Yerevan, the Armenian capital, to Tbilisi.

Georgian officials said the pair pleaded guilty after a closed hearing on Monday and agreed to co-operate with police, in a case that has highlighted the black market sale of nuclear material in former Soviet states.

The region, which lies on the fringes of Russia's Caucasus, has porous borders, widespread corruption and unknown quantities of unsecured materials left over from the Soviet period.

According to media reports, the uranium discovered on the two men was nearly 90 per cent enriched and usable in a nuclear warhead.

'More material'

The two men had attempted to sell the uranium for $1.5m to an agent they believed represented an extreme Islamic group, but turned out to be an undercover police officer.

The pair were reportedly attempting to sell the uranium as a sample and said they were able to obtain more.

Some officials are concerned that despite the small amount uncovered, more uranium could still be available on the black market.

"The dangerous thing is that there might be more material out there somewhere," Archil Pavlenishvili, chief of Georgia's nuclear smuggling unit, told the AP news agency.

"This proves that if a criminal or an extremist is wealthy enough, it is possible to obtain material."

Shota Utiashvili, Georgia's interior minister, called the operation "a big success for our nuclear smuggling unit," which has received $50m from the US in recent years to help it combat trafficking in atomic materials.

He told the AFP news agency that Mikheil Saakashvili, Georgia's president, had told world leaders about the case at a nuclear summit in Washington in April, but the full details could not be released until the men had pleaded guilty.

Tonoian and Ohanian are facing at least 10 years in prison. According to officials they are being tried in secret to protect the identity of the undercover officers involved in the case.

The case is the third involving the smuggling of nuclear materials to be uncovered in Georgia, a former Soviet republic on Russia's southern border closely allied to the US.

In a joint US-Georgia sting operation in 2006, officials arrested a Russian citizen trying to sell 100 grammes of highly enriched uranium to a Georgian officer posing as a buyer from an extreme 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.

Washington has signed agreements with a number of ex-Soviet states under its Smuggling Outreach Initiative, which is aimed at preventing nuclear trafficking, including Ukraine, Kazakhstan, Georgia and Kyrgyzstan.