"In addition, five Orlan-10 unmanned aerial vehicles [UAVs] are performing the task of air monitoring the surface situation," the ministry said.
The defense ministry added that the Black Sea Fleet's Epron rescue vessel with 16 divers and the Tiger telecontrolled deep-water vehicle is en route to the rescue operation area.
Two of Russian Geographical Society's manned deep-sea vehicles are also expected to arrive in the vicinity of the crash site for search operations soon, the ministry added.
A search and rescue operation in the area of Russia's Tu-154 aircraft crash is conducted in three shifts, with the number of people involved boosted up to 3,500 people, the Russian Defense Ministry said.
"The number of rescue team members involved in the search and rescue operation in the area of the aircraft crash is increasing. The operation is conducted in three shifts. The number of those involved increased up to 3,500 people," the statement said.
On Sunday morning, the Russian Defense Ministry's Tu-154 plane heading from Moscow to Syria with 92 people on board, including the Alexandrov Ensemble members, and nine journalists, disappeared from radar shortly after refueling and take-off from the Russian southern city of Adler, near Sochi. Fragments of the plane were discovered 1.5 kilometers (1 mile) from the sea shore of Sochi at a depth of 50-70 meters (164-184 feet).
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