The black box of a Russian military plane that crashed in the Black Sea shortly after take off from Sochi, was found on Tuesday, the Defence Ministry announced.

The Tupolev Tu-154 plane's black box was located at a depth of 17 metres in the Black Sea by a FALCON underwater drone, RT News reported the ministry as saying.

The Tu-154 was carrying 84 passengers and eight crew members. It disappeared from the radar on Sunday morning 20 minutes after taking off from Sochi Adler Airport. It crashed into the Black Sea en route to Latakia, Syria, where Russia has a military base.

At least 12 bodies from the plane crash have been recovered so far, the ministry said.

Also on Tuesday, five more fragments of the crashed plane, including the fuselage and parts of the engine, were located.

Some 45 ships, 15 underwater drones, 192 divers, 12 planes, and five helicopters are currently involved in the search, according to the ministry.

The black box will be delivered within the next few hours to the Central Research Institute of the Air Force of Defence Ministry in the city of Lyubertsy, a Moscow suburb.

A source told TASS earlier that one of the key theories as to why the plane crashed "is that foreign objects penetrated the engine," adding that other possible causes, such as pilot error and technical failure, are also being investigated.

The victims included 64 members of the world-famous Alexandrov Ensemble army choir, humanitarian workers and several media crews that were scheduled to take part in Christmas and New Year's celebrations at Latakia, Syria.

Russia on Monday observed a nationwide day of mourning.