Azerbaijan Airlines flight J2-8243 on Wednesday had flown hundreds of kilometres off its scheduled route to crash on the opposite shore of the Caspian Sea, after what Russia's aviation watchdog said was an emergency that may have been caused by a bird strike.
But an aviation expert suggested that cause seemed unlikely.
Officials did not immediately explain why it had crossed the sea, but the crash came shortly after drone strikes hit southern Russia.
Drone activity has shut airports in the area in the past and the nearest Russian airport on the plane's flight path was closed on Wednesday morning.
Azerbaijan President Ilham Aliyev said according to information he had received, the plane changed course due to poor weather.
But he added the cause of the crash was unknown and must be fully investigated.
"This is a great tragedy that has become a tremendous sorrow for the Azerbaijani people," he said.
Video of the crash showed the plane descending rapidly before bursting into flames as it hit the seashore, and thick black smoke then rising. Bloodied and bruised passengers could be seen stumbling from a piece of the fuselage that had remained intact.
Reuters was able to verify from visible landmarks that the video was filmed on the Caspian shore near Aktau.
Sixty-two passengers and five crew were aboard.
The death toll was disclosed by Deputy Prime Minister Kanat Bozumbayev in a meeting with an Azerbaijani delegation in Aktau, Russian news agency Interfax reported.
Kazakhstan's emergencies ministry said in a statement that fire services had put out the blaze and that the survivors, including two children, were being treated at a nearby hospital.
The cause of the plane crash is unknown but investigations are set to get underway shortly. (EPA PHOTO)
The bodies of the dead were being recovered.
Azerbaijan Airlines said the Embraer 190 jet was flying from Baku to Grozny, the capital of the Chechnya region in southern Russia, but had been forced to make an emergency landing around three kilometres from Aktau in Kazakhstan.
"Preliminary: after a collision with birds, due to an emergency situation on board, its commander decided to go to an alternate airfield - Aktau was chosen," Russia's aviation watchdog said on Telegram.
But a collision with birds typically results in the plane landing in the nearest available field, said Richard Aboulafia, an analyst at consultancy AeroDynamic Advisory.
"You can lose control of the plane, but you don't fly wildly off course as a consequence."
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