TBILISI (Reuters) - The security service in the former Soviet republic of Georgia said on Thursday it had detained five Georgian citizens who were trying to sell $3 million worth of radioactive uranium.

Security service officers did not say whether the group had a buyer for the uranium, nor where the group had acquired it.

World leaders have been concerned about the security of Soviet nuclear weapons since the Soviet Union's demise in 1991. Concern has also grown that radical groups are seeking material with which to make a 'dirty bomb'.

"The detainees were planning to sell nuclear material with total weights of 1 kilogram and 665 grams, which contained two radioactive isotopes - Uranium-238 and a small amount, 0.23 percent, of Uranium-235," security service investigator Savle Motiashvili told a briefing.

Motiashvili added that given the gamma ray emission, direct and long-term exposure to the substances was dangerous for life and health.

A Tbilisi city court put the group into pre-trial custody. They face five to 10 years in prison if found guilty.

Georgia's security service has foiled several attempts to sell uranium or other radioactive materials.

Earlier this month, they detained six Georgian and Armenian citizens who were trying to sell $200 million worth of the uranium-238 isotope.

In 2006, a resident of Russia's North Ossetia region was arrested for trying to sell weapons-grade uranium for $1 million to agents he thought were radical Islamists. He was sentenced to 8 1/2 years in prison.

(Reporting by Margarita Antidze; Editing by Vladimir Soldatkin and Toby Chopra)