Russia's FSB security service said on Monday it had thwarted an assassination attempt on Sergei Aksyonov, the Russian-backed head of Crimea, the Interfax news agency reported.
The FSB said it had detained a Russian man who had been hired and trained by Ukraine's security services to kill Aksyonov by blowing up his car.
There was no immediate comment from Ukraine.
Russia annexed Crimea, where it bases its Black Sea Fleet, from Ukraine in 2014.