Kirillov, who was chief of Russia's Nuclear, Biological and Chemical Protection Troops, was killed outside his apartment building along with his assistant when a bomb hidden in an electric scooter went off.
He was the most senior Russian military officer to be assassinated inside Russia by Ukraine. Ukraine's SBU intelligence service, which accused Kirillov of being responsible for the use of chemical weapons against Ukrainian troops, something Moscow denies, took responsibility for the killing.
Russian Lieutenant General Igor Kirillov was killed outside an apartment building in Moscow. (AP PHOTO)
Russia's Investigative Committee, which probes serious crimes, said in a statement on Wednesday that the unnamed suspect had told them he had come to Moscow to carry out an assignment for Ukraine's intelligence services.
In a video of the confession published by the Baza news outlet, which is known to have sources in Russian law enforcement circles, the suspect is seen sitting in a van describing his actions.
It was not clear what conditions he was speaking in and Reuters could not immediately verify the video's authenticity.
Dressed in a winter coat, the suspect is shown saying he had come to Moscow at the orders of Ukraine's intelligence services, bought an electric scooter, and then received an improvised explosive device to carry out the hit months later.
He describes how he had placed the device on the electric scooter which he had parked outside the entrance of the apartment block where Kirillov lived.
The suspect, who is thought to have been born in 1995, is shown saying he remotely detonated the device when Kirillov left the building. He says Ukraine had offered him $US100,000 and residency in a European country.
Investigators said they were identifying other people involved, and the daily Kommersant newspaper reported that one other suspect had been detained. Reuters could not independently confirm the report.
Maria Zakharova, a spokeswoman for Russia's Foreign Ministry, said Moscow would raise the assassination at a session of the United Nations Security Council on December 20.
Everyone involved in the killing would be found and punished, and Moscow would not be intimidated, she said.
"We see that the Kyiv regime has taken responsibility once again for a new terrorist attack. All these SBU losers and the mad Kiev regime are all tools managed by the Anglo-Saxons," Zakharova said, using a term Russia uses to describe the United States and Britain.
"They are the main beneficiaries of Kiev's terrorism."
Russian President Vladimir Putin is yet to comment on the killing of Kirillov. (AP PHOTO)
The US State Department said Washington had no connection to the killing or any prior knowledge of it. A spokesperson for British Prime Minister Keir Starmer said Kirillov had "propagated an illegal invasion and imposed suffering and death on the Ukrainian people."
Russian President Vladimir Putin, who says Moscow's actions in Ukraine are designed to protect Russia's security against NATO as it expands, has not commented publicly on the killing.
Dmitry Peskov, his spokesperson, praised the work of the Russian intelligence services in investigating the case and accused Ukraine of using "terrorist methods."
Former president Dmitry Medvedev criticised an editorial in the London Times newspaper which called the killing "a legitimate act of defence by a threatened nation".
He said the same logic would imply Western officials providing Ukraine with military aid would be legitimate targets for Russia.
Moscow holds Ukraine responsible for a series of killings on its soil. Ukraine says Russia's war against it poses an existential threat to the Ukrainian state and has made clear it regards such killings - intended to weaken morale and punish those Kyiv regards guilty of war crimes - to be legitimate.
