KIEV, Ukraine — A prominent commander of the Russian-backed separatist forces in eastern Ukraine has been killed by a bomb, rebel leaders and Russian news reports said on Monday.

Arsen Pavlov, the commander, who went by the nom de guerre Motorola after the brand of walkie-talkie he preferred, was blown up as he rode the elevator in his apartment building on Sunday in Donetsk, the larger of the rebels’ two urban strongholds in eastern Ukraine, Russian news accounts said.

Each side blames the other for the killing: Ukrainian officials said Russian special forces had been purging the charismatic but unpredictable early leaders of the rebel movement, while the separatists said Ukrainian assassins were operating behind their lines.

Mr. Pavlov is the latest of about half a dozen commanders from the 2014 insurgency to be killed far from the front line. Interfax, a Russian news agency, cited a separatist official as saying that the elevator Mr. Pavlov had been using had been rigged with an improvised explosive device.