Georgian police have arrested four people in the capital Tbilisi for trying to sell radioactive substance cesium-137, the interior ministry said Friday.

"We discovered that somebody was offering cesium-137 for sale, conducted an operation and seized the substance," ministry spokesman Shota Utiashvili told AFP.

"Four people were arrested," he said, adding that cesium-137 was a "fairly common substance" and that the operation had been routine.

He could not specify how much of the material had been seized.

Cesium-137 is a by-product of nuclear fission that is used on industrial sites and for medical devices, but could also be used to make dirty bombs with the potential to spread radiation.

The arrests follow the announcement by Georgian police this month that they had arrested two Armenian men in March accused of smuggling 18 grammes (0.6 ounces) of highly enriched uranium from Armenia into Georgia and trying to sell it to an undercover agent posing as an Islamic extremist.

The case 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 or radioactive weapons.

It was the third case of smuggling of nuclear materials to be uncovered in Georgia, an ex-Soviet republic on Russia's southern border which has received millions of dollars in US aid to combat trafficking in nuclear materials.