Witnesses say the plane was intact as it fell so the most likely explanation is that it flew into a thunderstorm.
A Russian airliner carrying 170 people crashed in flames on Tuesday in eastern Ukraine, probably after hitting turbulence, killing all on board, officials said.
"There are witnesses who said the plane was intact as it fell so the most likely explanation is that it flew into a thunderstorm," Irina Andrianova, a spokesman for Russia's Emergencies Ministry, told First Channel television.
The ministry said there were no survivors. Vasily Nalyotenko, deputy head of Pulkovo Airlines, which operated the Soviet-designed Tu-154, said 170 people were on board, including 10 crew and 39 children.
Ukrainian officials said helicopters circling the crash site about 45 km north of the regional town of Donetsk saw the plane in flames. Bad weather in the area was still hampering rescue efforts.
Flight 612 took off from the Black Sea resort of Anapa and was bound for its home base of St Petersburg. Its route went across Ukraine's eastern tip.
"The aircraft issued an SOS at 15.37 (Moscow time - 1137 GMT). At 15.39, it disappeared from radar screens," Russia's Emergencies Ministry said.
The Russian Transport Ministry said the crew reported severe turbulence in a distress message sent from 11,000 meters (35,000 feet) before disappearing. It said the plane came down near a village north of Donetsk.
A spokesman for the Ukrainian Emergencies Ministry told Fifth Channel television in Kiev that a fire may have broken out in the plane, but Russian officials disputed this.
The Tu-154, dating from Soviet times, is the workhorse of most airlines operating in ex-Soviet states.
Airlines operating in former Soviet republics initially had a patchy safety record in the aftermath of the collapse of communism, but this has improved in recent years.
However the crash was the second involving a regional Russian airline this year.
Last month, an Airbus A-310 belonging to Sibir airlines crashed and burst into flames after veering off the runway in the Siberian city of Irkutsk, killing 122 people.
In May 113 people, including Russians, died when an Airbus A-320 belonging to Armenian airline Armavia crashed on its way from Yerevan to the Russian resort of Sochi.