KYIV -- A separatist commander in the eastern Ukrainian region of Donetsk has been assassinated, the Russia-backed separatists say.

Mikhail Tolstykh, better known by his nom de guerre Givi, died after a bomb exploded in his office in the separatist stronghold of Donetsk just after 6 a.m. local time on February 8, de facto separatist authorities and a source with ties to its military told RFE/RL by phone.

The separatists announced two days of mourning.

Donetsk separatist leaders called the killing a "terrorist" attack organized by the Ukrainian intelligence services.

"The Ukrainians...cannot defeat us on the battlefield, so they kill us in a malicious way," separatist leader Aleksandr Zakharchenko said.

The Ukrainian Security Service (SBU) denied it was responsible, instead suspecting an internal operation to clear the rebel ranks.

"People tied to illegal armed groups are purged by special agencies beyond the line of contact," Yuriy Tandit, an adviser of the SBU chief, told the 112 Ukrainian channel.

Kremlin spokesman Dmitry Peskov called the killing "an attempt to destabilize the situation" in eastern Ukraine, where fighting between government forces and Russia-backed separatists has killed more than 9,750 people since April 2014.

Givi, 36, was leading the Somali battalion and made a name for himself in the brutal battle for Donetsk airport.

Video footage from 2015 shows him verbally and physically abusing Ukrainian servicemen captured during the battle.

Another separatist commander -- Arseny Pavlov, who was better known by the nickname Motorola -- died on October 16 when a bomb exploded in an elevator in his apartment block in Donetsk.

With reporting by AFP, RIA Novosti, TASS, and Interfax