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The head of the Russian army's chemical weapons division was killed on Tuesday in a brazen attack in Moscow claimed by Kyiv -- the most senior military figure assassinated in Russia yet as the Kremlin's campaign in Ukraine drags on.

Igor Kirillov was killed along with his assistant when an explosive device attached to a scooter went off outside an apartment building in southeastern Moscow, Russian and Ukrainian officials said.

The attack took place in a residential area in the capital a day after President Vladimir Putin boasted of Russian troop successes in Ukraine, nearly three years after the Kremlin sent soldiers into its pro-Western neighbour.

Kirillov, 54, was the head of the Russian army's chemical, biological and radiological weapons unit and in October was sanctioned by Britain over the alleged use of chemical weapons in Ukraine.

A source in Ukraine's SBU security service told AFP it was behind the early morning explosion in what it called a "special operation", calling Kirillov a "war criminal."

Russia's Investigative Committee said that an "explosive device planted in a scooter parked near the entrance of a residential building was activated on the morning of December 17 on Ryazansky Avenue in Moscow."

The blast shattered several windows of the building and severely damaged the front door, according to an AFP reporter on the scene.

Russian authorities said they were probing the attack as "terrorism".

Ukraine's SBU alledged Kirillov was responsible for using banned chemical weapons on the battlefield.

"Kirillov was a war criminal and an absolutely legitimate target, as he gave orders to use banned chemical weapons against the Ukrainian military," the SBU source, who spoke on condition of anonymity, told AFP.

"Such an inglorious end awaits all those who kill Ukrainians. Retribution for war crimes is inevitable," the source added.

Ìý'Banned chemical weapons'Ìý

There have been assassinations on Russian territory before, but such attacks in Moscow -- where fighting in Ukraine often feels distant -- are rare.

Residents AFP spoke to said they had initially assumed the loud noise they heard came from a nearby construction site.

Student Mikhail Mashkov, who lives in the building next door, said he was woken up by a "very loud explosion noise", thinking "something fell at the construction site", before looking outside.

Home-maker Olga Bogomolova told AFP she thought a container had fallen at the construction site but then realised "it was a very strong explosion", saw "broken windows" and that it was something else.

Previous targets included nationalist writer Darya Dugina -- killed in a car bomb attack outside Moscow in 2022 -- and pro-conflict military correspondent Maxim Fomin -- blown up in a Saint Petersburg cafe in 2023.

But Kirillov is the most senior Russian military official to be killed.

He had been in the post of head of the Russian military's Radiological, Chemical and Biological Defence unit since 2017.

The unit does not oversee Russia's nuclear weapons.

Kyiv had a day earlier charged Kirillov in absentia on allegations of committing "war crimes" against Ukraine.

"The official is responsible for the massive use of banned chemical weapons," Ukraine's SBU Security Service said on Monday, alleging more than 4,800 cases of Russia using chemical munitions since February 2022.

Britain and the United States have accused Russia of using the toxic agent chloropicrin against Ukrainian troops in violation of the Chemical Weapons Convention (CWC).

Chloropicrin is an oily liquid with a pungent odour known as a choking agent that was widely used during World War I as a form of tear gas.

The Organisation for the Prohibition of Chemical Weapons (OPCW) specifically prohibits its use.

The UK government in October slapped sanctions on Kirillov and his unit "for helping deploy these barbaric weapons", charges that Moscow has denied.

Russia has said it no longer possesses a military chemical arsenal but the country faces pressure for more transparency over the alleged use of toxic weapons.

In lengthy televised briefings, Kirillov had regularly accused Kyiv and the West of running secret networks of bio-labs that were developing banned chemical agents across Ukraine -- claims rejected by the West and independent fact-checking organisations.

The killing comes a day after Putin hailed 2024 as a "landmark year" for its military offensive on Ukraine, saying his troops had the upper hand across the entire front line.