Since it's the first thing everyone thinks these days when they hear about plane-crashes-- no, there is no reported terrorism link.
There are conflicting reports of what happened on board before the plan attempted an emergency landing.
Minutes later, the Russian Emergency Situations Ministry said wreckage from the plane was found on the ground.
The Interfax news agency quoted Ukrainian Emergency Situations Ministry spokesman Igor Krol as saying a fire broke out on the plane at 32,800 feet and that the crew decided to try to make an emergency landing. However, it also quoted Russian aviation official Alexander Neradko as saying that the plane might have run into strong turbulence.
Russian Emergency Situations Ministry spokeswoman Irina Andriyanova said she received information that "the plane most likely was hit by lightning."
The plane's landing gear failed to deploy normally and the aircraft crashed "on its belly," Mr Krol told the French news agency AFP.
But Russian officials told the Itar-Tass news agency that the plane had encountered severe weather conditions before it came down.
Ms Andrianova said the available information suggested "the plane most likely was hit by lightning".
Update: Plane crashed two minutes after an SOS signal.
The three-engine Tupolev Tu-154 is the workhorse of Russia's commercial fleet, carrying about half of all the country's air passengers.
Designed as the Soviet counterpart to the Boeing 727 and the European-made Trident, about 1000 were made.
It has had a chequered history and has been involved in at least 30 air disasters since it went into service in 1968.
Let me not ever aspire to cruising altitude in the "Soviet counterpart" of anything. A sad story.