Ukraine's security service on Monday said it had arrested Russian agents who were planning to assassinate the defence minister and the military intelligence chief.

The SBU "arrested killers from the Russian special services who were plotting the assassinations" of Defence Minister Oleksiy Reznikov and military intelligence boss Kyrylo Budanov, the service said on Telegram.

It published a video showing an armed group subduing and handcuffing two men in civilian clothing who were approaching a car.

The arrests took place in Kovel in northwestern Ukraine, with one of the alleged plotters arriving in the country from Russia via Belarus, the SBU said.

It claimed the men were preparing "the physical liquidation" of the two top officials and an unnamed "Ukrainian activist".

Each killing was to be rewarded with a sum of money ranging from $100,000 to $150,000, the SBU added.