Russia's FSB security service said on Monday it had thwarted a Ukrainian assassination attempt on Sergei Aksyonov, the Russian-backed head of Crimea, arresting a Ukrainian agent before he was able to blow up Aksyonov's car.
There was no immediate comment from Ukraine on the Russian allegation.
It came amid Russian media reports that security has been stepped up in Crimea, the Black Sea peninsula which Russia annexed from Ukraine in 2014, and that additional security checks are being made on people wanting to cross a bridge from Russia's southern Krasnodar region into Crimea.
The FSB said in a statement that it had arrested a Russian national recruited as an assassin by Ukraine's SBU intelligence agency who had undergone explosives, reconnaissance and sabotage training in Ukraine.
His plan, it said, had been to blow up Aksyonov's car, but he had been detained as he tried to retrieve an explosive device from a hiding place.
The FSB did not name the arrested man, who it said was in his mid-thirties. It said he had entered Crimea in June and that it was investigating him on suspicion of "attempting to commit a terrorist act" and "of illegally possessing explosives."
Aksyonov thanked the FSB for preventing what he called an attempt on the lives of the "Republic of Crimea's leadership" and said he was sure that the individuals who ordered the assassination would be found and punished.
Pledging to retake Crimea
Ukraine has pledged to retake full control of Crimea, the headquarters of Russia's Black Sea Fleet, as well as large areas of eastern and southern Ukraine that Russia has captured since launching what it calls its "special military operation" last year, something Kyiv and the West call a brutal war of conquest.
Russia's Izvestia newspaper reported on Monday that a 13 km (8-mile)-long traffic jam had formed at the entrance in Russia's southern Krasnodar region to the bridge to Crimea. It said that Crimean transport officials had increased the number of checkpoints around the bridge on Sunday.