Amid concerns of a stalemate in the war between Russia and Ukraine and pressure for a negotiated peace, Kyiv’s assassination of a top general at Moscow on Tuesday marks a show of defiance.

Lieutenant General Igor Kirillov was killed by a bomb hidden in a scooter outside his apartment building at Moscow, a day after Ukraine’s security service leveled criminal charges against him. A Ukrainian official said the service carried out the attack.

Kirillov, the chief of the military’s nuclear, biological and chemical protection forces, was killed as he left for his office. Kirillov’s assistant also died in the attack.

Kirillov, 54, was under sanctions from several countries, including the U.K. and Canada, for his actions in Moscow’s war in Ukraine. On Monday, Ukraine’s Security Service, or SBU, opened a criminal investigation against him, accusing him of directing the use of banned chemical weapons.

An official with the SBU said the agency was behind the attack. The official, who spoke on condition of anonymity because they were not authorized to release the information, described Kirillov as a “war criminal and an entirely legitimate target.”

The SBU has said it recorded more than 4,800 occasions when Russia used chemical weapons on the battlefield since its full-scale invasion in February 2022. In May, the American State Department said that it had recorded the use of chloropicrin, a poison gas first deployed in World War I, against Ukrainian troops.

Russia has denied using any chemical weapons in Ukraine and, in turn, has accused Kyiv of using toxic agents in combat.

Kirillov, who took his current job in 2017, was one of the most high-profile figures to level those accusations. He held numerous briefings to accuse the Ukrainian military of using toxic agents and planning to launch attacks with radioactive substances — claims that Ukraine and its Western allies rejected as propaganda.

The bomb used in Tuesday’s attack was triggered remotely, according to Russian news reports. Images from the scene showed shattered windows and scorched brickwork.

Russia’s top state investigative agency said it’s investigating Kirillov’s death as a case of terrorism, and officials in Moscow vowed to punish Ukraine.

The deputy head of Russia’s Security Council, Dmitry Medvedev, described the attack as an attempt by Kyiv to distract public attention away from its military failures and vowed that its “senior military-political leadership will face inevitable retribution.”

The security council is led by President Putin.