Investigators last night recovered both the flight data recorder and the voice recorder from the wreckage of a Russian passenger plane that crashed into a Ukrainian field after apparently being hit by lightning during a severe thunderstorm, killing all 170 people aboard.

The flight recorders of the Pulkovo Airlines Tu-154 will be vital to explain the cause of the third fatal crash this year of a Russian passenger airliner.

Emergency officials said preliminary information suggested that weather caused the crash about 45km from the city of Donetsk in eastern Ukraine.

The plane was flying to St Petersburg from the Black Sea resort of Anapa, a holiday destination popular with Russian families, when it got into trouble over Ukraine.

"Right now, it is difficult to determine the cause of the accident," Ukraine's Transport Minister Mykola Rudkovsky said.

But he noted the weather was severe and suggested the plane might have struck a cyclone.

Ukrainian officials said a storm with heavy winds, driving rain and flashes of lightning was raging through the region at the time.

Russian Emergency Situations Ministry spokeswoman Irina Andrianova, citing information from her Ukrainian counterparts, said the plane was likely hit by lightning.

The Russian Emergencies Ministry said there were no survivors. Grieving relatives, preparing to fly from St Petersburg to the crash site overnight, were being offered counselling and sedatives at St Petersburg's airport, an airline official said.

Russian President Vladimir Putin declared today a national day of mourning for the victims, who an airline official said included 10 crew and 39 children. The dead included Dutch nationals, but most were Russian.

The smouldering wreckage was strewn across a gully and woodland outside the eastern Ukrainian village of Sukha Balka.

The crash was Russia's third big air disaster since May, making this the deadliest year on record for air travel in the former Soviet Union and raising fresh concerns about regional airlines' aircraft maintenance and pilot training. Air-traffic control data showed the crew had reduced altitude sharply after the plane encountered problems. They may then have tried to crash land but the jet slammed into the ground and exploded, witnesses said.

"An SOS was issued from 11,700m and then again at 3000m," Pulkovo deputy chairman Anatoly Samoshin said. "There was an incomprehensible sentence. We didn't understand what was said." At 3000m, communication ceased.

The Tu-154, dating from Soviet times, is the workhorse of most airlines in ex-Soviet states. Post-Soviet airlines had a patchy safety record in the aftermath of the collapse of communism, but it has improved in recent years.