MOSCOW -- A senior Ukrainian official says the killer of Kremlin critic Denis Voronenkov, who was gunned down in Kyiv, was a Russian agent.

Anton Gerashchenko, an adviser to Ukraine's interior minister, identified the man who shot Voronenkov on Thursday as 28-year old Pavel Parshov and said he had been trained in Russia by Russian security services.

"He underwent a special course at a school for saboteurs," Gerashchenko wrote Friday in a Facebook post.

Dmitry Peskov, a spokesman for Russian President Vladimir Putin, told reporters that Gerashchenko's allegation was "absurd."

Voronenkov, a former Russian lawmaker who became a vociferous critic of Moscow following his recent move to Ukraine, was shot dead near the entrance of an upscale hotel in the centre of the Ukrainian capital. He was wanted on criminal charges in Russia.

The murder ignited a war of words between Moscow and Kyiv, with Ukrainian President Petro Poroshenko immediately calling the killing an "act of state terrorism." Russian officials angrily denied the charge and suggested Ukraine was organizing a coverup.