Russia's internal spy agency said on Thursday it had detained two Ukrainian saboteurs plotting to blow up the power lines of two nuclear power stations to shut down the reactors and embarrass Russia on the eve of this month's Victory Day holiday.

The Federal Security Service (FSB), the main successor to the Soviet-era KGB, said that the saboteurs working for Ukrainian foreign intelligence had laid explosives on a total of 11 pylons of the Leningrad and Kalinin nuclear power stations.

"The plan of the Ukrainian special services was to prompt the shutdown of the nuclear reactors, disrupt the operation of the nuclear power plants and cause serious economic and reputation damage to the Russian Federation," the FSB said in a statement.

The attacks were due to take place on the eve of the May 9 anniversary of victory over Nazi Germany, the FSB said. It did not say when the men had been arrested.

The Leningrad station is Russia's biggest atomic power plant. It is located on Gulf of Finland close to St Petersburg. The Kalinin nuclear power station is 350 km north of Moscow.

The FSB said the saboteurs were recruited in 2022 by Ukraine's foreign intelligence service (FISU) and received special training at camps in Kyiv and the Mykolaiv region. They entered Russia via Poland and Belarus, the FSB said.

Two Russian accomplices were also detained, the FSB said.

The explosives - including 36.6 kg of C-4 plastic explosives, 61 detonators and 38 timers - came via Poland, Lithuania and Belarus, the FSB said.

"The defendants have confessed to cooperating with the foreign intelligence service of Ukraine in order to prepare and commit sabotage on the territory of Russia," the FSB said. (Reporting by Guy Faulconbridge Editing by Peter Graff)