Ukraine's SBU intelligence service on Tuesday claimed the bomb attack that killed a top Russian general in Moscow, who they accused of using chemical weapons against Ukrainian troops.
Lieutenant General Igor Kirillov, who was chief of Russia's Nuclear, Biological and Chemical Protection Troops, was killed outside an apartment building along with his assistant when a bomb hidden in an electric scooter went off, Russia's Investigative Committee, which probes serious crimes, said.
An SBU source confirmed to Reuters that the Ukrainian intelligence agency had been behind the hit. "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.
The source said that a scooter containing explosives was detonated, killing both Kirillov and his aide, as they stepped out of a building on Ryazansky Prospekt in Moscow.
Unverified video footage of the attack circulating on social media showed two men exiting the building to get into a car followed by a large explosion as the two men remained on the pavement. Reuters could not independently verify the footage.
Kirillov, 54, is the most senior Russian military officer to be assassinated inside Russia by Ukraine and his murder is likely to prompt the Russian authorities to review security protocols for the army's top brass.
Former President Dmitry Medvedev, now a senior Russian security official, told a meeting shown on state TV that Moscow would avenge what he called an act of terrorism.
"Law enforcement agencies must find the killers in Russia," said Medvedev. "Everything must be done to destroy the masterminds (of the killing) who are in Kyiv. We know who these masterminds are. They are the military and political leadership of Ukraine," he said.
Foreign Ministry spokeswoman Maria Zakharova, speaking to Russian news agencies, dismissed a comment from the U.S. State Department that Washington had no connection to the killing or any prior knowledge of it.
The United States, she said, "created the Kyiv regime, sponsors it, provides money and sends weapons endlessly. The proof is clear: Washington has not once condemned a single terrorist act or planned murder committed by the Kyiv regime."
There was no immediate comment from President Vladimir Putin.
Moscow holds Ukraine responsible for a string of high-profile assassinations on its soil designed to weaken morale and punish those Kyiv regards guilty of war crimes. Ukraine, which says Russia's war against it poses an existential threat to the Ukrainian state, has made clear it regards such targeted killings as a legitimate tool.
Reuters photographs and video from the scene showed a shattered entrance to an apartment building with bomb-blackened bricks and the doors hanging off their hinges and what looked like two bodies lying beneath black plastic sheets on the snow.
Russia denies Ukrainian allegations it uses chemical weapons on the battlefield and Kirillov, who was married with two sons, was himself sometimes shown on state TV giving briefings at the Defence Ministry in which he accused Ukraine of violating nuclear safety protocols or the West of various alleged crimes.
Britain in October imposed sanctions on Kirillov and his nuclear defence forces for using riot control agents and over multiple reports of the use of the toxic choking agent chloropicrin on the battlefield.
Such agents, Ukraine has alleged, are used to disorient its troops, leaving them unable to defend themselves against Russian attacks.
Sergei Sitnikov, a regional Russian governor, said Kirillov was his friend and had told him he was aware of a threat against him.
"Some time ago, he told me that he had already been warned that the hunt for him had begun," Sitnikov said in a statement, saying he believed Kyiv wanted to kill Kirillov for various reasons, including his involvement in the development and use of a heavy flamethrower system.
Kirillov was murdered a day after Ukrainian state prosecutors charged him in absentia with the alleged use of banned chemical weapons, the Kyiv Independent cited the SBU as saying.