The murder of Arseny Pavlov threatens to destabilise an already fragile ceasefire in Ukraine's troubled east.
A pro-Ukrainian neo-Nazi group has claimed responsibility for the assassination of a pro-Russian rebel commander in the country's troubled east on Sunday.
The group that claimed responsibility for the murder have already warned in a video uploaded to the Internet that they will next target the leaders of Ukraine's two breakaway regions - Luhansk People's Republic leader Igor Plotnitsky and Donetsk People's Republic "prime minister" Alexander Zakharchenko.
In response, Zakharchenko vowed revenge for the killing, which he said amounted to a declaration of war by Ukraine, throwing a ceasefire between Ukrainian government forces and pro-Russian separatists into doubt.
The already fragile ceasefire has been shaken by sporadic clashes since it was enforced after an agreement was signed in Minsk in February 2015.
Thirty-three-year-old Russian national Arseny Pavlov, better known by his nickname "Motorola," was killed along with his bodyguard in an explosion in his apartment building in the self-proclaimed Donetsk People's Republic, the rebel-controlled region's "deputy defence minister" Eduard Basurin said.
"The bomb was placed above the lift's cabin. The blast was concentrated inside the cabin," Basurin informed Reuters. "We had intelligence from our agent networks that diversionary groups were being sent our way to kill prominent military and political leaders.
Ukraine's SBU security service refused to comment on the assassination, but the country's Ukrainian Interior Ministry spokesman Artyom Shevchenko said on social media that Pavlov was killed in rebel infighting.
"It looks like he was eliminated by his cousins in crime of this anti-human terrorist gang," Shevchenko said. "This is a sad but just end."
Ukraine has been gripped by a pro-Russian insurgency in its eastern Donbass region since April 2014. The rebellion was triggered after former president Viktor Yanukovych was forced to flee his presidential palace after months of violent protests in Kiev against his rule.
Yanukovych had fallen out of favour in the former Soviet republic after he decided to forgo an Association Agreement with the European Union, instead opting for closer economic ties with Russia.
His government was replaced with a pro-EU administration, now led by Petro Poroshenko. Since assuming office in June 2014, Poroshenko has boosted Ukraine's cooperation with the EU as he seeks the West's help in pressuring Russia to back off from its support for pro-Russian rebels and end its annexation of the Crimean peninsula.
Ukrainian authorities have also had to deal with the rise of neo-Nazi activity in the country, which has largely come about as a reaction to Russian aggression. In March 2014, as pro-Russian troops were occupying Crimea, the leader of the Ukrainian far-right militant group Right Sector, Oleksandr Muzychko, was killed during a confrontation with police.
Last year, a grenade was set off outside the Ukrainian parliament during a far-right protest against the decentralisation of power, a move taken appease rebel demands as part of the peace process brokered by France, Germany and Russia. The blast killed a national guardsman and injured 125 others.