Russia's Federal Security Service (FSB) said on Saturday it had foiled a plot by Ukraine to kill a high-ranking Russian officer and a pro-Russian war blogger with a bomb hidden in a portable music speaker.

The FSB, the main successor to the Soviet-era KGB, said that a Russian citizen had established contact with an officer from Ukraine's GUR military intelligence agency through the Telegram messaging application.

On the instructions of the Ukrainian intelligence officer, the Russian citizen had then retrieved a bomb from a hiding place in Moscow, the FSB said. The bomb, equivalent to 1 1/2 kg of TNT and packed with ball bearings, was concealed in a portable music speaker, the FSB said.

The FSB did not name the officer or the blogger who was the target of the plot. Ukraine's GUR military intelligence agency could not be immediately reached for comment.

Ukraine says Russia's war against it poses an existential threat to the Ukrainian state and has made clear it regards targeted killings - intended to weaken morale and punish those Kyiv regards guilty of war crimes - as legitimate.

Russia has said they amount to illegal "acts of terrorism" and accuses Ukraine of assassinating civilians such as Darya Dugina, the daughter of a nationalist ideologue, in 2022.

On Dec. 17, Ukraine's SBU intelligence service killed Lieutenant General Kirillov, chief of Russia's Nuclear, Biological and Chemical Protection Troops, in Moscow outside his apartment building by detonating a bomb attached to an electric scooter. Kyiv had accused him of promoting the use of banned checmial weapons, something Moscow denies.

Donald Trump's designated Ukraine envoy, retired Lieutenant-General Keith Kellogg, told

Fox News

on Dec. 18 that such killings were "not really smart" and going "a little bit too far."

Russia said that it would take revenge for the Kirillov killing.