Search operations have been stepped up to recover the bodies of victims from Sunday's plane crash in the Black Sea.

Investigators are also trying to locate the aircraft's black box. A spokesperson for Russia's defense ministry said rescuers have been working around-the-clock and additional personnel have joined in the search.

The Kremlin said investigators have found no evidence of a terrorist attack after the TU-154 aircraft plunged into the Black Sea 2 minutes after taking off from the southern Russian city of Sochi.

All 84 passengers and eight crew members are believed to have died in the incident. Passengers included members of the Red Army Choir who were en route to entertain Russian troops stationed in Syria.