The plane with some 91 people onboard, including 83 passengers and eight crew members lost contact with land while flying over the Black Sea, the source said.
Journalists, military personnel, musicians from Alexandrov Ensemble, an official army choir of the Russian armed forces, were on board.
Initially it was reported that the aircraft was carrying some 70 people.
"According to preliminary data, Russian Defense Ministry's Tu-154 disappeared from the radar screens after departing from Sochi. About 70 people were onboard," a source said to the agency on Sunday.
The priliminary data shows that the disappeared plane was heading to Syria's Hmeimim airbase. According to the source, the possible reasons for the alleged crash could be technical malfunction or pilot error.
The source added that the plane disappeared from radars 20 minutes after departing from Sochi, Krasnodar Krai.
"The plane disappeared from radars about 20 minutes after takeoff: it departed [from Sochi] at 5.20 AM local time and the contact was lost at about 5.40 AM."
All rescue services of Krasnodar Krai and neighboring regions are engaged in the search operation.