* Plane crashed into Black Sea minutes after take-off
* No reports of survivors as search continues
* Most likely causes seen as technical or pilot error
The Russian Defence Ministry said one of its TU-154 planes had disappeared from radar screens at 5:25 a.m. (0225 GMT), two minutes after taking off from Adler in southern Russia, where it had stopped to refuel from Moscow, en route to Syria.
An unnamed ministry source told Russian news agencies that those on board had almost no chance of surviving and that no life rafts had been found. Interfax cited another unnamed source as saying the plane had not sent an SOS signal.
The jet, a Soviet-era design first introduced in the 1970s, had been carrying 84 passengers and eight crew members, Major-General Igor Konashenkov, a ministry spokesman told reporters.
At least 60 were members of the Red Army Choir, also known as the Alexandrov Ensemble, who were being flown out to Russia's Hmeymim air base in Syria to entertain troops in the run-up to the New Year.
Nine Russian reporters had also been on board as well as military servicemen.
Konashenkov said fragments of the plane had been found at a depth of about 70 metres (yards) in the Black Sea about 1.5 km (1 mile) off the coast near the city of Sochi.
"The search operation is continuing," said Konashenkov. "Four ships, five helicopters and a drone are working in the area," he said, saying a military commission had flown to Sochi to look into what happened.
The Interfax news agency cited an unnamed source in the emergency services as saying about six bodies had already been recovered from the sea.
Russia's RIA news agency, citing an unidentified security source, said preliminary information indicated that the plane had crashed because of a technical malfunction or a pilot error. Another source told Russian agencies that the possibility of a militant act had been ruled out. The weather had been good.
According to the defence ministry's list of passengers, Elizaveta Glinka, a member of Putin's advisory human rights council, was on the plane. Her mobile phone was switched off when Reuters called her on Sunday.
Kremlin spokesman Dmitry Peskov told reporters on Sunday it was too early to say what had caused the crash. President Vladimir Putin was being kept constantly informed of the latest developments, Peskov said.
Russian military investigators said in a statement they had opened a criminal investigation into the crash.
The Kremlin said Putin expressed his deepest condolences to those who had lost loved ones in the crash and ordered Prime Minister Dmitry Medvedev to head a government investigatory commission into the crash.
Russia's Defence Ministry regularly flies musicians into Syria to put on concerts for military personnel. The base they were heading for, Hmeymim, is in Latakia province. It is from there that Russia launches air strikes against Syrian rebels.
A Russian military jet crashed in Siberia with 39 people on board as it tried to make an emergency landing near a Soviet-era military base. Nobody was killed in that incident, though 32 people were airlifted to hospital.
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