MOSCOW/KIEV (Reuters) - A prominent Russian separatist commander was assassinated in eastern Ukraine on Sunday evening, his allies said on Monday, accusing Ukrainian government forces of murdering him to try to destabilize an already fragile ceasefire.

Flowers are left in memory of battalion commander of the separatist self-proclaimed Donetsk People's Republic Arseny Pavlov, who goes by the nom de guerre"Motorola", near the site of assassination in Donetsk, Ukraine, October 17, 2016. REUTERS/Alexander Ermochenko

Arseny Pavlov, a Russian national who went by the nom de guerre “Motorola”, was blown up in the lift of his apartment building in Donetsk together with his bodyguard, according to Eduard Basurin, deputy defense minister in the government of the self-styled Donetsk People’s Republic.

Donetsk is the biggest city in the Ukrainian region of Donbass, which pro-Russian militants seized in 2014 and where daily skirmishes and shellings test a shaky 2015 ceasefire deal.

“The bomb was placed above the lift’s cabin. The blast was concentrated inside the cabin,” Basurin told Reuters.

“We had intelligence from our agent networks that diversionary groups were being sent our way to kill prominent military and political leaders.”

Four armed masked men claimed responsibility for the killing in a video uploaded to the Internet. The men, who appeared alongside a flag belonging to a pro-Ukrainian neo-Nazi group, said they would next target Igor Plotnitsky, leader of the self-proclaimed Luhansk People’s Republic, together with Alexander Zakharchenko, prime minister of the Donetsk People’s Republic.

Zakharchenko said the murder amounted to a declaration of war by Ukraine and promised revenge.

Pavlov, 33, was born in Russia, and commanded the separatist “Sparta” battalion which fought Ukrainian government forces in several key battles, including the struggle for Donetsk airport.

Short and sporting a ginger beard, pro-Kremlin media and his fellow separatists cast him as a hero, but in Ukraine, many regarded him as a hate figure and a symbol of Russian expansionism.

In a 2015 telephone interview with the Kyiv Post newspaper, a man who identified himself as Pavlov was recorded as saying he had murdered 15 Ukrainian prisoners of war.

The Donetsk News Agency, an official separatist news outlet, cited Denis Pushilin, a senior pro-Russian separatist leader, as saying Pavlov’s killing was a Ukrainian attempt “to rock the situation.”

A spokeswoman for Ukraine’s SBU security service declined to comment, but some Ukrainian officials suggested the separatist commander had been killed in rebel infighting.

“It looks like he was eliminated by his cousins in crime of this anti-human terrorist gang,” Ukrainian Interior Ministry spokesman Artyom Shevchenko said on social media. “This is a sad but just end.”