• Russia's head of chemical weapons was killed on Tuesday when a scooter bomb exploded in Moscow.
• Ukraine was behind the attack, a Ukrainian Security Service source told BI.
• Igor Kirillov is the most prominent military official to be killed since Russia invaded Ukraine.

A high-ranking Russian general responsible for Russia's chemical weapons was killed on Tuesday by a bomb placed in a scooter on a Moscow street.

A source inside Ukraine's Security Service with knowledge of the attack told Business Insider the agency was behind the death of Lieutenant General Igor Kirillov.

The Wall Street Journal also reported Ukrainian officials as saying the killing was a special operation by the Security Service of Ukraine.

Kirillov, the head of Russia's Nuclear, Biological, and Chemical Protection Troops, was killed by a bomb planted in a scooter parked on a street in Moscow, Russia's investigative committee said in a statement on Telegram on Tuesday.

"According to the investigation, on the morning of December 17, an explosive device was detonated in a scooter parked next to the entrance of a residential building on Ryazansky Prospekt in Moscow," the statement said.

"As a result of the incident, the head of the radiation, chemical and biological protection troops of the Armed Forces of the Russian Federation Igor Kirillov and his assistant were killed," it added.

The committee said it had opened a criminal case and that investigators and forensic experts were working at the scene.

The Russian investigations committee didn't immediately respond to a request for comment from BI.

According to the Ukrainian source, the bomb on the scooter was detonated when Kirillov and his assistant were entering a nearby house on Ryazansky Prospekt. BI couldn't independently verify the claim.

"Kirillov was a war criminal and an absolutely legitimate target," they said, accusing Kirillov of giving orders to use banned chemical weapons against Ukrainian forces.

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

Kirillov is the most prominent military official to be killed since Russia launched its full-scale invasion of Ukraine in 2022, according to the Financial Times.

He was sanctioned by the UK in October for the use of chemical weapons in Ukraine, including the choking agent chloropicrin.

Kirillov's death follows a string of similar attacks, some of which sources said were the work of Ukraine's Security Service or other agencies.

Dmitry Medvedev, deputy chair of Russia's Security Council and a former Russian president, described Kirillov's killing as a terrorist attack, and offered his condolences to Kirillov's family, per the TASS news agency.

He also said that Ukraine would pay for its actions.

This is a developing story. Check back for updates.

• Russia is firing 1.5x more shells than Ukraine, Western officials said, down from 10x earlier this year.
• While its artillery advantage is shrinking, glide bombs are compensating, officials told Sky News.
• Russian advances on the front lines have come at a terrible cost, they said.

Russia's artillery advantage over Ukraine is shrinking fast.

Russia's advantage is now down to 1.5 rounds for every shell Ukraine fires back, Sky News reported, citing Western officials. Previous Ukrainian estimates put its artillery advantage at 10 to 1 earlier this year, and as high as 15 to 1 in the early months of its full-scale invasion in 2022.

The unnamed Western officials said a "wide variety of factors " were behind the drop, including constraints on Russia's production lines, challenges transferring rounds to the frontline via rail, drone strikes against major Russian and North Korean ammunition depots, and Western ammunition supplies to Ukraine.

But they told the outlet that Russia seems to be compensating by dropping huge amounts of glide bombs on the front lines.

One official said Russia's "massive" increase in glide bomb use was having a "devastating effect."

Since the start of the war, Russia has frequently targeted Ukraine with glide bombs — cheap but highly destructive weapons that are notoriously difficult to intercept.

Warfare and airpower analysts have said that Ukraine's ability to counter these threats is limited, as moving its best air-defense systems closer to the front lines makes them vulnerable to attack.

Until last month, Ukraine was not given permission to use Western-supplied long-range missiles to strike bases inside Russia, from which many of the attacks originate.

Ukraine has responded by making its own glide bombs, using Western-provided bombs and fitting them to its F-16 fighter jets. It dropped some inside Kursk during its surprise cross-border raid into the region in August.

Ukrainian forces have also targeted Russian aircraft capable of dropping glide bombs, and have used drones to strike military bases storing the weapons.

But the glide bombs have wreaked havoc on Ukraine's infrastructure.

Russia dropped more than 900 glide bombs on Ukraine in just a single week at the end of October and early November, Ukrainian President Volodymyr Zelenskyy said at the time.

Russia also continues its incremental advance in Ukraine.

Late last month, analysts from the Institute for the Study of War said Russian forces were advancing at their fastest rate in Ukraine's east since the early months of the conflict.

But this has come at a terrible cost, the Western officials told Sky News, with one comparing Russia's front-line losses to those seen at the Battle of the Somme, one of the largest and bloodiest battles of World War I.

November saw Russia experience a new record for the average number of dead and wounded per day in the war, according to the UK Ministry of Defence, which said in an intelligence update this week that the losses were "likely reflective of the higher tempo of Russian operations" against Ukraine.

It was the third month in a row that Russia suffered record-breaking daily losses, it said.