US President Donald Trump has offered his Russian counterpart Vladimir Putin his condolences by phone for the fatal crash of a Russian passenger plane at the weekend, Russian news agencies cited the Kremlin as saying.
All 71 people on board were killed.
Meanwhile hundreds of emergency workers in Russia scoured snow-covered fields outside Moscow on Monday, collecting body fragments and clues.
Putin ordered a special commission to investigate what caused the AN-148 plane operated by Saratov Airlines to crash outside Moscow shortly after taking off for the city of Orsk about 1500km southeast of the capital.
Among the possible causes investigators are looking into are weather, human error and the plane's technical condition. The crew did not send any distress signals.
Experts on Monday started studying the plane's two flight recorders, as well as some of the around 500 plane fragments recovered so far. Footage from a CCTV camera that captured some of what happened was posted online on Monday. It showed what looked like a large ball of fire streaking through the sky.
Investigators, who have opened a criminal case into the tragedy, said in a statement however that the plane had been in one piece when it plummeted towards the ground and not on fire. An explosion had occurred only after it crashed, they said.
Debris and human remains were spread over a radius of one kilometre around the crash site and recovery teams had found more than 1400 body fragments so far, the RIA news agency reported, saying the search could carry on for a week.
Officials from Russia's Emergency Situations Ministry told a meeting broadcast on state TV on Monday that DNA tests were being organised with the relatives of those killed to try to identify body fragments.
The plane, manufactured in 2010, had been carrying 65 passengers and six crew. The passenger list showed many young people were on board, including a five-year-old girl.
Mourners left flowers, soft toys and photographs at Orsk airport on Monday.
The plane's operator, Saratov Airlines, said in a statement on Monday it was temporarily grounding its fleet of AN-148 planes until investigators had established what happened.
It said the plane involved in the crash had undergone thorough checks last month and that it had successfully flow