Ukrainian President Petro Poroshenko described the killing of Denis Voronenkov as an act of Russian "state terrorism" while Moscow dismissed the allegation as "absurd."
A former lawmaker of the Russian State Duma, Denis Voronenkov, was shot and killed by an assailant armed with pistol in the centre of the Ukrainian capital of Kiev on Thursday, police said.
The attacker, whose identity was not disclosed, also died later in a hospital after being fatally injured by Voronenkov's bodyguard, police spokesman Artiom Shevchenko said.
Kiev police chief Andriy Kryshchenko said Voronenkov was killed around noon local time (1000 GMT) and his bodyguard was also injured in the attack outside a hotel in Kiev.
Voronenkov fled to Ukraine last year and was helping the Ukrainian authorities build a treason case against former leader Viktor Yanukovich, Ukraine's pro-Russian former president.
He had also spoken out against Russia's annexation of Crimea in March 2014, although he voted for the move at the time.
TRT World's Nafisa Latic reports.
Russian "state terrorism"
Ukrainian President Petro Poroshenko described the shooting as an act of Russian "state terrorism."
"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," Poroshenko said.
Poroshenko said it was "no accident" that Voronenkov was shot on the same day as a warehouse storing tank ammunition was blown up at a Ukrainian military base.
Nobody was hurt, but some 20,000 people were evacuated following several explosions in the city, about 100 kilometres from the frontline of Ukraine's war against Russian-backed rebels.
TRT World spoke to journalist Volodymyr Solohub who is following the developments in Kiev.
Russia dismiss allegation as "absurd"
Moscow denied any involvement in Voronenkov's murder.
"We believe that all the falsehoods that can already be heard about much-hyped Russian involvement are absurd," Kremlin spokesman Dmitry Peskov was quoted as saying about the killing.
Relations between Kiev and Moscow are at an all-time low after Russia annexed the Crimean peninsula in March 2014 and the subsequent outbreak of Russian-backed separatist fighting in Ukraine's eastern Donbass region, which has killed more than 10,000 people.