"A terrorist attack is ruled out," Irina Andriyanova said. "Ukrainian sources said the plane was caught in a thunderstorm."
The St. Petersburg-based Pulkovo Airline said the passengers of the Tu-154 jet, the mainstay of the Russian air fleet, included 39 children under the age of 12 and six children under the age of two.
Russian television's Channel One said citing Ukrainian sources that the plane had caught fire at an altitude of about 33,000 feet (10,000 meters) and the crew had decided to make an emergency landing after giving an SOS signal.
Ukrainian emergencies services told Russian television that a heavy thunderstorm could have caused the crash.
The Tu-154 was flying from the Russian resort of Anapa on the Black Sea to St. Petersburg over Ukrainian territory. It was expected to land in St. Petersburg at 17:45 Moscow time (13:45 GMT).
Andriyanova also said the plane had come down far away from residential areas 45 kilometers (30 miles) north of the major city of Donetsk.
She said that 32 Russian rescuers would fly to the site at 19:00 Moscow time (15:00 GMT).
