Armenia's security services said Friday that they had arrested four men for selling radioactive strontium to a foreign buyer.

The four Armenian citizens were detained "at the moment of handing over radioactive strontium to a foreigner," the National Security Service said in a brief statement without specifying the nationality or identity of the alleged foreign buyer.

They have been charged with the illegal possession and sale of radioactive materials, the statement said.

The case drew fresh attention to concerns that unsecured nuclear materials around the former Soviet Union could be sold to violent extremists.

Last year three Armenians were arrested for allegedly smuggling enriched uranium into neighbouring Georgia, which they tried to sell to an undercover officer posing as a buyer for Islamic radicals.

Georgian police in 2010 also arrested four people for allegedly trying to sell another radioactive substance, cesium-137.