The main black box of the Syria-bound Russian military plane that crashed into the Black Sea with 92 people onboard has been found in a massive ongoing search operation, authorities said Tuesday.

"The main black box was found at 5:42 am Moscow time (0242 GMT) 1,600 metres from the shore at a depth of 17 metres," the defence ministry said in a statement, adding that the device would be sent to the Moscow region to be deciphered by experts.

The Tu-154 jet, whose passengers included more than 60 members of the internationally renowned Red Army Choir, was heading to Russia's military airbase in Syria when it went down off the coast of the resort city of Sochi shortly after take-off on Sunday.

Investigators have yet to confirm the cause of the crash, but officials have said that an act of terror was not being considered as a possible explanation.

The finding of the black box comes as searchers are scrambling to recover bodies and remaining debris from the aircraft in an operation involving 192 divers, 45 vessels, 12 planes, five helicopters and 15 deepwater machines.

The defence ministry said that five plane fragments were found overnight 30 metres (98 feet) under water at around 1,700 metres from the shore.

Part of the plane's fuselage, measuring 3.5 metres by 4.5 metres, was recovered Monday evening.

Four scenarios

Russia's federal security service has said it is looking into four suspected causes of the crash, which include pilot error, technical failure, faulty fuel and an object in the engine.

The defence ministry said Tuesday that 12 bodies and 156 body fragments had been recovered from the water since the crash. They will be sent to Moscow for identification, it said.

The Kommersant daily newspaper reported that investigators are relying on a witness statement by a coast guard member who saw the plane in its final moments descending toward the sea with its nose tilted sharply upward.

Authorities have not said how long it would take to decipher the black box, which could provide information on the reasons for the crash.

Lost choir

The Tu-154 jet went down on Sunday morning minutes after taking off at 5:25 am (0225 GMT) from Sochi's airport, where it had stopped to refuel after flying out from the Chkalovsky military aerodrome in the Moscow region.

The FSB said all passengers remained on the plane when it stopped in Sochi, with one customs officer and one border guard coming on board as it was being fuelled.

Onboard were 64 members of the Alexandrov Ensemble -- the army's official musical group, also known as the Red Army Choir -- and their conductor Valery Khalilov.

The choir was set to perform for Russian troops at the Hmeimim airbase in Syria, which has been used to launch air strikes in support of Moscow's ally President Bashar al-Assad.

Other passengers included military officers, journalists and popular charity worker Yelizaveta Glinka, affectionately known as "Doctor Liza", who had been set to delivery medical supplies to a hospital in the coastal Syrian city of Latakia near the Russian airbase.

Russia observed a day of mourning Monday, cancelling entertainment programmes.

People brought flowers to improvised memorials at the port in central Sochi and the city's airport, as well as to the Moscow headquarters of the Red Army Choir and the office of Fair Aid, the NGO that Glinka headed, which primarily worked with Moscow's homeless.

Tu-154 aircraft have been involved in a number of accidents in the past, including the April 2010 crash killing then-Polish president Lech Kaczynski and his delegation. They are no longer used by commercial airlines in Russia.