MOSCOW/KIEV (Reuters) - Russia's Federal Security Service said on Thursday it had detained a group of Ukrainian saboteurs who it accused of planning to attack military sites and infrastructure in Crimea, but Kiev dismissed the story as false.

The Federal Security Service, or FSB, said it had confiscated explosive devices and weapons from the group, which it said was made up of intelligence agents from the Ukrainian Defence Ministry.

The Interfax news agency cited a person familiar with the matter as saying that the FSB had detained three Ukrainian military personnel who had been plotting to attack power plants and water purification facilities in Crimea.

Russia annexed Crimea from Ukraine in 2014.

Ukraine said the FSB story was false.

"Nobody has detained Ukrainian military personnel," Andriy Lysenko, a spokesman for the National Security and Defence Council, told Reuters. "This is yet another frame-up by the Russian security forces."

The incident echoes a similar episode in August when Russia said it had detained a group of Ukrainian saboteurs and dismantled a Ukrainian spy network inside Crimea. Kiev said that incident never happened either.

Real or imagined, the August episode stirred tensions between Moscow and Kiev with Russian President Vladimir Putin accusing Ukraine of using terrorist tactics to try to provoke a new conflict.

(Reporting by Maria Kiselyova in Moscow and Natalia Zinets in Kiev; Writing by Andrey Ostroukh; Editing by Andrew Osborn)