KYIV -- An unruly commander of Russia-backed separatist forces in eastern Ukraine who once boasted about having killed captive soldiers has died in Donetsk in an apparent assassination, the latest in a spate of killings that the separatists blame on Kyiv but that others suspect have been carried out by Russia.
A top Donetsk separatist leader vowed revenge, sparking concerns of new violence by separatists whose war with Kyiv's forces has killed more than 9,600 people in eastern Ukraine since April 2014.
Arseny Pavlov, a Russian citizen better known by his nom de guerre, Motorola, succumbed to "injuries incompatible with life" after an improvised explosive device detonated as he entered the elevator of his apartment building in the separatist stronghold of Donetsk on October 16, the Interfax news agency reported. It said a bodyguard also died in the blast.
The separatist leadership in Donetsk confirmed Pavlov's death and said an investigation was under way. Aleksandr Zakharchenko, head of the separatist group that calls itself the Donetsk People's Republic, blamed the Ukrainian authorities for the blast that killed Pavlov, who headed a fighting force called the Sparta battalion.
Zakharchenko said the killing amounted to a "declaration of war" by President Petro Poroshenko and vowed to retaliate against members of Ukraine's military and security services, as well as their families.
"All officers, lieutenant colonels, majors who operate on our territory, all your agents, families, are beyond the law as of now. Not only here but in Zhytomyr, Kyiv, Kharkiv, Dnipropetrovsk," Zakharchenko told a press conference in Donetsk, listing cities that are under government control. "And I would add: when we come to your home, there will be no mercy towards you, believe me."
WATCH: Aleksandr Zakharchenko On 'Motorola' Killing
Hours after Pavlov's death was reported, an unverified video showing four masked, armed men taking credit for the killing was shared online by Russian media and separatist supporters.
Standing in front of a Ukrainian flag as well as one representing a neo-Nazi group with roots in Ukraine and Russia, one man vows to come after Zakharchenko and Luhansk separatist leader Ihor Plotnitsky next, before the four men make a Nazi salute. The authenticity of the video could not be confirmed, and Russian media have used faked footage in the past during the conflict.
'Sad But Fair'
Neither Poroshenko nor his office immediately commented on Pavlov's death or responded to Zakharchenko's remarks. But Artem Shevchenko, director of communications for Ukraine's Interior Ministry, said Pavlov got what he deserved.
"It is a sad but fair end of such bastards on our Ukrainian land," Shevchenko wrote in a post on Facebook.
Other Ukrainians, many of whom view Pavlov as a war criminal, reacted on social media with relief and even delight.
Pavlov, 33, was born in Russia's northern Komi Republic and was reported to have once lived in Rostov-on-Don, near the Ukrainian border. He was married with two children, who were reportedly inside the family's Donetsk apartment at the time of the blast. The marriage, celebrated with guns and roses in Donetsk, was rumored to be his second and conducted without him having divorced his first wife.
Media reports say he fought in the second war between Russian forces and Chechen separatists, which began in 1999, and had worked as a lifeguard and a car washer.
In 2014, Pavlov traveled to the eastern Ukrainian city of Slovyansk to fight in an anti-Kyiv force led by Igor Girkin, another Russian who played a key role in the early stages of the conflict.
Pavlov later went on to fight in fierce battles against Ukrainian forces in Ilovaysk in August 2014 and Debaltseve in February 2015 -- separatist campaigns that Western intelligence and reporters on the ground said were backed by regular Russian forces.
Pavlov also led the Sparta battalion in the protracted battle for Donetsk airport, which left a once-gleaming new terminal completely destroyed, and boasted about having killed 15 Ukrainian combatants captured by his men there.
In another 2015 interview with RFE/RL's Current Time TV, however, Pavlov said he didn't kill the Ukrainian prisoners.
WATCH: Motorola Pushes Back On Execution Accusations
Nevertheless, Amnesty International has accused Pavlov of war crimes and the European Union imposed sanctions on him for his alleged actions in Ukraine.
There had been at least one reported attempt on Pavlov's life, on June 24, when a vehicle packed with explosives blew up outside a trauma center in Donetsk where he was reportedly receiving treatment for a battlefield injury. No casualties were reported.
Pavlov is one of several separatist military leaders to have been killed in apparent assassinations, but the first in the Donetsk region. Several in Luhansk -- the other province held in part by the separatists -- have been killed or found dead under mysterious circumstances. Others in Donetsk, including Zakharchenko, have been the target of assassination attempts.
Possible Moscow Role?
While the separatists have blamed Kyiv in all cases, many analysts believe Moscow could be behind such killings.
"Moscow may want to give the [separatist groups] more acceptable civilian faces not directly implicated in MH17 or war crimes," Alex Kokcharov, a Ukraine and Russia risk analyst at the London-based defense and security consultancy IHS Markit, told RFE/RL.
An international probe found that a Russian missile fired from separatist-held territory downed Malaysia Airlines flight MH17 in July 2014, killing all 298 people on board.
Kokcharov said that Russia might hope removing those seen as responsible could "put more pressure on Kyiv" to make concessions in efforts to end the conflict and resolve the status of the separatist-held areas under the Minsk agreement, signed in February 2015.
Kokcharov suggested the killing could further reduce the chances of implementation of the already frayed Minsk deal and lead to an increase in fighting. Regardless of who killed Pavlov, he said, his death "gives a pretext for [the separatists] to accuse Kyiv and escalate the conflict."
Using a colloquial name for the Donetsk and Luhansk regions, a pro-Kremlin Russian analyst, Igor Korotchenko, said on Twitter: "In light of recent events, a new round of war in the Donbas, unfortunately, becomes almost inevitable."
Russian and Ukrainian leaders, along with their French and German counterparts, are considering meeting in Berlin on October 19 to seek progress on the Minsk agreement, but Moscow and Kyiv are locked in a stalemate over how and when the deal should be implemented.