Russia's Chief of State Security Service (FSB) warned that terrorists were seeking access to nuclear materials across the former Soviet Union.
Alexander Bortnikov, Chief of FSB and main successor to the Soviet-era KGB, gave no further details about the attempts or which groups had sought the materials.
"We have information which indicates that terrorists are continuing to attempt to get access to nuclear materials as well as biological and chemical components," he was quoted as saying by Interfax and Itar-Tass.
"We are paying constant attention to this issue," Bortnikov said, referring to concerns that terrorists could get their hands on nuclear materials.
Nuclear experts say there is no sign that terrorists have acquired weapons-grade nuclear material but there have been at least 18 documented cases of theft or loss of plutonium or highly enriched uranium, several in the ex-Soviet Union.
Despite major, heavily US-funded security improvements at Russian facilities containing nuclear materials since the 1991 Soviet collapse, experts say the risk of theft remains.
A recent report by Bunn commissioned by the US-based Nuclear Threat Initiative said the highest risks of nuclear theft today were in Pakistan and Russia, which has the largest stockpiles.