"A sabotage group from the Ukrainian foreign intelligence service... tried to blow up some 30 power lines of nuclear power plants in Leningrad and Kalinin" in early May with the aim of stopping the nuclear reactors at the plants, Russian news agencies quoted the FSB as saying in a statement.

The aim was to cause "serious economic harm to Russia and damage its reputation", it said.

The men had brought down "a pylon carrying a high voltage line and planted mines in four pylons carrying high voltage lines from the Leningrad nuclear power station".

Explosives were also planted near seven similar pylons linked to the Kalinin nuclear plant, it added.

The FSB said it was looking for a third man with Ukrainian-Russian nationality, adding that 36.5 kilos of explosives and about 60 detonators were found in the homes of the suspects and their alleged accomplices.

Two Russians who allegedly helped the trio were also arrested, the FSB said.