"Technical means for protection of Russian nuclear power stations are the best in the world," Alexander Rumyantsev, head of the Russian atomic energy agency, said at a press conference.

"But there are problems with protection of other nuclear sites," he said, apparently referring to laboratories and research centers, most of which were built in the Soviet era and some of which sprawl over entire closed towns dotted around the country.

"We are talking about very large areas and very large cities, so there are always problems," Rumyantsev said, a rare admission from a senior government official of security deficiencies in the country's nuclear facilities.

He added however that despite these problems, which he did not specify, Russia's nuclear facilities were well-enough guarded "to repel any threat."

Earlier this year, the US National Intelligence Council published a report asserting that theft of nuclear material in Russia "has occurred" and stating that Russian nuclear facilities were too vulnerable to attack.

Russian officials have said on numerous occasions that no unauthorized persons could acquire material in Russia that could be used for producing a nuclear weapon or a radioactive "dirty bomb."