Cesium-137 has a wide range of industrial uses and is employed in radiotherapy. But it could also be combined with explosives to create a radiological bomb with the potential to spread radioactive contamination.

The seizure follows the arrest in Georgia in March of two Armenian men accused of smuggling 18 grams of highly-enriched uranium from Armenia into Georgia, both former Soviet republics.

A Georgian Interior Ministry spokesman said a “radioactive seizure” had been made in a downtown residential district of the capital, Tbilisi, and four people were in custody.

He gave no further details, but private Georgian broadcaster Imedi, which is close to the government, said the substance was believed to be cesium-137.

At least two similar smuggling cases have been reported since 2003, underscoring the risks posed by unsecured nuclear material being smuggled across porous borders in the former Soviet Union and potentially falling into the hands of radical groups.

Georgia, a pro-Western country of 4.5 million people sandwiched between Russia and Turkey, has received millions of dollars in aid from the United States in recent years to help combat smuggling of nuclear material.