According to Ukraine, a senior Russian general named Lt Gen Igor Kirillov was killed in a Moscow explosion.
The explosive device was reportedly hidden in an electric scooter and detonated outside an apartment building in the Russian capital.
Ukraine has claimed the attack, which marks one of the boldest targeted assassinations of a senior military official since Russia’s full-scale invasion of the country.
Kirillov is the head of the military’s chemical, biological, and radiological weapons unit.
He died along with his assistant when the blast went off as the two men left a building in a residential area in southeast Moscow on Tuesday.
A source in Ukraine’s SBU security service said Kyiv was behind the attack.
Kirillov is the most senior Russian military official to be killed in an assassination away from the frontline since the start of the Russian offensive in Ukraine nearly three years ago.
Speaking at a meeting with senior Russian leadership, Dmitry Medvedev, the deputy head of the country’s security council, said Moscow would avenge the killing.
“We must do everything to eliminate those who ordered the assassination of Gen. Kirillov, namely the military-political leadership of Ukraine,” he said.