A 20,000-strong rescue team has reportedly been sent by the Russian authorities to the Ural Mountains after a meteor struck.
Some 1,100 people were injured after the meteor blazed across the western Siberian sky with a blinding flash and booming shockwave.
The Russian Academy of Sciences said it entered the Earth's atmosphere at 33,000mph (54,000kph) - 15 times the speed of a rifle bullet - and shattered into pieces around 18-32 miles (30-50km) high in the sky.
It reportedly exploded with the force of 20 atomic bombs.
Nasa estimated its speed was even faster at 40,000mph, and said it exploded at about 12-15 miles high and left a trail 300 miles long.
The meteor weighed an estimated 7,000 tons and was about the size of a bus, Nasa said.
Fragments of at least one meteorite were seen falling from the sky in the Chelyabinsk region at around 9am local time on Friday, around 930 miles (1,500km) east of Moscow.
The fall of such a large meteor estimated as weighing dozens of tonnes and the size of a double-decker bus, was extremely rare, while the number of casualties as a consequence of its burning up around a heavily-inhabited area was unprecedented.
"There was panic. People had no idea what was happening," said Sergey Hametov, of Chelyabinsk.
Many were hurt by flying glass as windows were blown in. Witnesses described feeling a pressure wave and hearing explosions overhead as the object hurtled to Earth.
Lessons had just started at Chelyabinsk schools when the meteor exploded, and officials said 258 children were among those injured. Amateur video showed a teacher speaking to her class as a powerful shock wave hit the room.
Schools were closed for the day and theatre shows cancelled across the region after the shock wave blew out windows amid temperatures as low as minus 18 degrees Celsius (zero degrees Fahrenheit).
There was no immediate word on deaths or anyone struck by rock fragments.
Meteors typically cause sonic booms when they enter the atmosphere because they are travelling much faster than the speed of sound.
The defence ministry said it had identified a six-metre crater at a lake in the region and has sent soldiers to the site of the apparent impact of one fragment.
Thousands of rescue workers were dispatched to help the injured and locate those needing help.
Many drivers in Russia have video recorders fitted in their cars in case of accidents or disputes with traffic police. Footage from these uploaded to YouTube show the fireball crashing to the ground.
The meteor explosion appears to be one of the most stunning cosmic events above Russia since the 1908 Tunguska Event, when a massive blast most scientists blame on an asteroid or a comet impact ripped through Siberia.
"I am scratching my head to think of anything in recorded history when that number of people have been indirectly injured by an object like this... it's very, very rare to have human casualties," Robert Massey, deputy executive secretary of Britain's Royal Astronomical Society (RAS), said.
There were reports of traffic in the Urals city of Chelyabinsk grinding to a halt and people seeking shelter in buildings as the fireballs lit up the sky.
Schools were closed for the day across the region and mobile phone networks were temporarily cut.
The meteor hit hours before the asteroid 2012 DA14 made the closest recorded pass of an asteroid to the Earth, although scientists said the two events were not linked.
Resident Anna Pinkus told Sky News she saw a bright light outside her window and then heard three explosions.
"It was a very loud sound," she said. "After that our windows began to shatter and shiver so it was very terrifying. First we thought it was a plane crash."
Planetary scientist Professor Ian Crawford of Birkbeck University said this was an unusual case as meteorite hits rarely cause any damage.
"Several times a year meteorites are observed to fall on the earth's surface but damage to people or property is very unusual," he said. "There are only several recorded cases of buildings being hit by meteorites."
Describing the course of the meteorite he said: "I think it's very likely that it would have been a larger lump of rock that broke up in the Earth's atmosphere - this is usually what happens.
"The rock comes in from space and hits the Earth's atmosphere. That decelerates it and puts a lot of stress on it. Then it's likely to fragment into lots of pieces."
The office of the local governor said that a meteorite had fallen into a lake outside the town of Chebarkul in the Chelyabinsk region and television images pointed to a six-metre (20-foot) hole in the frozen lake's ice.
However it has yet to be finally confirmed if meteorite fragments made contact with the Earth and there were no reports that any locals had been hurt directly by a falling piece of meteorite.
Chelyabinsk is Russia's industrial heartland, home to many factories and other huge facilities that include a nuclear power plant and the Mayak atomic waste storage and treatment centre.
A spokesman for Rosatom, the Russian nuclear energy state corporation, said that its operations remained unaffected.