Vigils have been held in Russia after a plane bound for Syria carrying 92 people crashed into the Black Sea shortly after takeoff.
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.
Backed by ships, helicopters and drones, rescue teams have been searching for the victims of Sunday's crash.
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.
The Russian defence ministry said it had recovered 10 bodies by late Sunday.
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 of the Soviet-built Tu-154 plane.
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 defence 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 defence ministry was taking its famed choir, the Alexandrov Ensemble, to a New Year's concert at Khmeimim airbase 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.
Syrian president Bashar al-Assad was among numerous foreign leaders who sent a condolence note to Putin, saying he received the news "with deep grief and sadness".
The Black Sea search area - which covered 10 square kilometres - was made more difficult by underwater currents that carried debris and body fragments into the open sea.
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 defence ministry.
|Rescuers unload fragments and remains of the Tu-154 plane crash near Sochi [EPA]
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 (ISIL) group claimed responsibility.
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 manoeuvrability 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.
|More than 60 members of the internationally-renowned Red Army Choir were on board the plane that crashed in the Black Sea [File:AFP]
Source: Al Jazeera and news agencies