A China Eastern passenger jet carrying 132 people crashed into a mountain in southern China on Monday.
The plane had dropped thousands of metres in just three minutes, and sparked a massive fire on impact.
The airline acknowledged that some aboard the Boeing 737-800 travelling from the city of Kunming to the southern hub of Guangzhou had died, but did not offer more specifics. President Xi Jinping quickly called for a full probe.
An unverified video carried by some Chinese media appeared to show a plane in a vertical nosedive. AFP could not immediately verify its authenticity.
Flight MU5735, which took off from Kunming shortly after 1pm (12pm Bangkok time), "lost airborne contact over Wuzhou" city in the Guangxi region on Monday afternoon, according to the Civil Aviation Administration of China (CAAC).
"The company expresses its deep condolences for the passengers and crew members who died in the plane crash," China Eastern said in a statement, without providing more information.
Hundreds of firefighters were dispatched to the scene in Teng county near Wuzhou, state media reported, as nearby villagers rushed to help the rescue effort.
Flight tracking website FlightRadar24 showed no data for the flight after 2.22pm.
The tracker showed the plane sharply dropped from 29,100 feet to 3,225 feet (8,870 to 982 metres) in three minutes, before flight information stopped.
Aerial images of the crash site showed a large crater in the side of a green mountainside. State TV footage followed orange-clad emergency workers gathering debris.
China Eastern changed its website to black and white on Monday afternoon.
The company said in a January report that it had 289 Boeing 737-series aircraft in its 751-strong fleet. Chinese media reported that the airline will now ground all the 737-800 jets.
