More than 3,000 people, including 109 divers, as well as ships, planes, helicopters and submersibles are involved in the operation near Sochi.
The Tu-154 plane – carrying soldiers, members of a famed army music ensemble and reporters – was heading for Syria.
All those on board the aircraft are feared dead after Sunday’s crash.
Monday has been declared a day of national mourning across Russia.
First bodies found
“As darkness fell, the operation in the crash area is continuing non-stop,” defence ministry spokesman Maj Gen Igor Konashenkov said at a briefing late on Sunday.
He said that powerful spotlights were being used overnight as the search teams were looking for plane debris.
Officials earlier said they were focusing on the area of 10.5 sq km (four square miles) just off the coast.
The defence ministry said in a statement: “Fragments of the Tu-154 plane of the Russian defence ministry were found 1.5km (one mile) from the Black Sea coast of the city of Sochi at a depth of 50m to 70m (165-230ft).”
Asked by reporters if it could have been a terrorist attack, Russian Transport Minister Maksim Sokolov said investigators were looking into an “entire spectrum” of possible causes of the crash.
But he stressed that it was too early to make any conclusions.
The plane disappeared from radar two minutes after taking off from Sochi’s Adler airport at 05:25 (02:25 GMT), heading for Latakia in Syria, the defence ministry said.
The flight had originated in Moscow and landed in Sochi for refuelling.
It was carrying 64 members of the famed Alexandrov military music ensemble, who were to perform for Russian troops in Syria.
