The news service said that Kirillov was killed in a “special operation,” citing a source in the SBU who described Kirillov as “an absolutely legitimate target.”

"The liquidation of the chief of the radiation and chemical protection troops of the Russian Federation is the work of the SBU," the source said.

Kirillov and his deputy approaching a residential building on the Russian capital’s Ryazansky Prospekt when an explosive device hidden in an electric scooter detonated, killing both men, the SBU source told Reuters.

Moscow has labeled the killing a “terrorist act,” the BBC reported. Russian state-controlled news agency Tass reported on Monday that the device was remotely operated and contained around 300 grams of TNT equivalent.

On Monday, Kirillov was charged in absentia by the SBU for his alleged role in the use of chemical weapons in Ukraine. The SBU claims Russia has used banned chemical weapons on the battlefield in Ukraine as many as 5,000 times.

"The pre-trial investigation is ongoing to document other facts of violations of international humanitarian law," the SBU said on Monday, as quoted by the BBC.

In October, the U.K. placed sanctions on Kirillov for overseeing the use of chemical weapons and spreading fake news. All his troops were also sanctioned in the move.