Russia called the allegation “absurd”.
Former MP Denis Voronenkov was killed by an assailant who was armed with a pistol and is now critically ill in hospital after being wounded by Voronenkov’s bodyguard, police said.
Voronenkov fled to Ukraine last year and was helping the Ukrainian authorities build a treason case against Yanukovich, Ukraine’s pro-Russia former president.
Voronenkov had also spoken out against Russia’s annexation of Crimea in March 2014, although he voted for the move at the time.
President Petro Poroshenko said the killing “is an act of state terrorism on the part of Russia, which (Voronenkov) was forced to leave for political reasons.”
“Voronenkov was one of the main witnesses of Russian aggression against Ukraine and, in particular, the role of Yanukovich regarding the deployment of Russian troops to Ukraine.”