A China Eastern Boeing 737-800 with 132 people on board has crashed in the southern province of Guangxi, officials said.
The Civil Aviation Administration of China (CAAC) said the crash occurred near the city of Wuzhou in Teng county.
The flight was travelling from Kunming in the western province of Yunnan to the industrial centre of Guangzhou along the east coast.
There was no immediate word on numbers of dead and injured.
The plane was carrying 123 passengers and nine crew members.
Chinese President Xi Jinping called for an "all-out effort" to be made in the rescue operation.
The People's Daily reported that 117 rescuers have already arrived at the crash site.
The Guangxi fire department is organising 650 rescuers who are heading to the site from three directions.
The CAAC said it has sent a team of officials, and the Guangxi fire service said work was under way to control a mountainside blaze ignited by the crash.
Satellite data from Nasa showed a massive fire in the area of where the plane went down at the time of the crash.
State media said local police first received calls from villagers alerting them to the crash at around 2.30pm (0630 GMT).
The Guangxi provincial emergency management department said contact with the plane was lost at 2.15pm (0615 GMT).
Chicago-based Boeing said it was aware of the initial reports of the crash and was "working to gather more information".
Shanghai-based China Eastern is one of the country's top three airlines, operating domestic and international routes serving 248 destinations.
China Eastern's flight number 5735 had been travelling at around 30,000ft when suddenly, just after 0620 GMT, the plane entered a deep dive at its cruising altitude speed of 455 knots (523mph, 842kph), according to data from flight-tracking website FlightRadar24.com.
The data suggests the plane crashed within a minute-and-a-half of whatever went wrong.
The plane stopped transmitting data just south west of the Chinese city of Wuzhou.
The aircraft was delivered to China Eastern from Boeing in June 2015 and had been flying for more than six years.
The twin-engine, single-aisle Boeing 737 is one of the world's most popular planes for short and medium-haul flights.
China Eastern operates multiple versions of the common aircraft, including the 737-800 and the 737 Max.
The deadliest crash involving a Boeing 737-800 came in January 2020, when Iran's paramilitary Revolutionary Guard accidentally shot down a Ukraine International Airlines flight, killing all 176 people on board.
The 737 Max version was grounded worldwide after two fatal crashes.
China's aviation regulator cleared that plane to return to service late last year, making the country the last major market to do so.
China's last fatal crash of a civilian airliner was in 2010.
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