Backed by ships, helicopters and drones, rescue teams have been searching for victims after a Russian plane carrying 92 people to Syria crashed into the Black Sea shortly after takeoff.

Investigators said on Sunday they were looking into every possible cause for the crash of the Soviet-built Tu-154 plane.

All 84 passengers and eight crew members on board the plane operated by the Russian military are believed to have died when it crashed two minutes after taking off at 5:25am local time (02:25 GMT) in good weather from the southern Russian city of Sochi.

Transport Minister Maxim Sokolov, in charge of a state probe into the crash, said on state television that investigators were looking into a "whole spectrum" of theories on the cause of the crash.

When asked if a "terror attack" could have been behind the crash, Sokolov said: "It is premature to speak of this."

He added that the aircraft's black boxes had yet to be found.

More than 3,000 people - including over 100 divers flown in from across Russia - worked from 32 ships and several helicopters to search the crash site, the defense ministry said.

Drones and submersibles were also being used to help spot bodies and debris. Powerful spotlights were brought in so the search could continue around the clock.

Emergency crews found fragments of the plane about 1.5km from shore. By Sunday evening, rescue teams had recovered 11 bodies and Sokolov, the transport minister, said fragments of other bodies were also found.

Famous doctor, choir on board

The plane belonging to the defense ministry was taking its famed choir, the Alexandrov Ensemble, to a New Year's concert at Hemeimeem air base in Syria's coastal province of Latakia.

Those on board also included nine Russian journalists and a Russian doctor famous for her work in war zones.

Yelizaveta Glinka has won wide acclaim for her charity work, which has included missions to war zones in eastern Ukraine and Syria. Her foundation said Glinka was accompanying a shipment of medicine for a hospital in Syria.

Russian President Vladimir Putin went on television to declare Monday a nationwide day of mourning.

"We will conduct a thorough investigation into the reasons and will do everything to support the victims' families," Putin said.

The Black Sea search area - which covered 10 square kilometers - was made more difficult by underwater currents that carried debris and body fragments into the open sea.

Transport Minister Sokolov said the plane's flight recorders did not have the radio beacons common in more modern aircraft, so locating them on the seabed was going to be challenging.

The Tu-154 is a Soviet-built three-engine airliner designed in the late 1960s. More than 1,000 have been built, and they have been used extensively in Russia and worldwide.

The plane that crashed on Sunday was built in 1983, and underwent factory check-ups and maintenance in 2014 and earlier this year, according to the defense ministry.

Investigators are exploring a number of possible causes to the crash, "including a catastrophic malfunction, a pilot error or a bird flying into the engine", said Al Jazeera's Natasha Ghoneim, reporting from Moscow.

Russian planes have been brought down previously by attacks. In October 2015, a Russian plane carrying mostly Russian tourists back from vacation in Egypt was brought down by a bomb over the Sinai Peninsula, killing all 224 people aboard. The local affiliate of the Islamic State of Iraq and the Levant group claimed responsibility.
Tu-154

Bashar Assad was among numerous foreign leaders who sent a condolence note to Putin, saying he received the news "with deep grief and sadness".

In recent years, Russian airlines have replaced their Tu-154s with more modern planes, but the military and other government agencies in Russia have continued to use them. While noisy and fuel-guzzling, the plane is popular with crews that appreciate its maneuverability and ruggedness.

Still, since 1994 there have been 17 major plane crashes involving the Tu-154 that have killed over 1,760 people. Most resulted from human error.

PHOTO CAPTION

Rescuers unload fragments and remains of the Tu-154 plane crash near Sochi [EPA]

Al-Jazeera