There was no immediate confirmation of the number of casualties, but the disaster prompted an unusually swift public reaction from President Xi Jinping, who said he was “shocked” and ordered an immediate investigation into its cause.
The airline “expresses its deep condolences for the passengers and crew members who died in the plane crash,” it said in a filing with the Shanghai Stock Exchange.
There was no word on the cause of the crash of the plane. State broadcaster CCTV said the crash “caused a mountain fire” which was later extinguished.
The Boeing 737-800 flight from the city of Kunming to the southern hub of Guangzhou “lost airborne contact over Wuzhou” in the Guangxi region on Monday afternoon, according to the Civil Aviation Administration of China (CAAC).
“This flight has crashed,” the CAAC said, adding the jet was carrying 123 passengers and nine crew members.
A Boeing spokesperson said “we are aware of the initial media reports and are working to gather more information.”
Hundreds of firefighters were dispatched to the scene in Teng county near the city of Wuzhou, state media reported, as nearby villagers rushed to help the rescue effort.
“Everyone went to the mountains,” Tang Min, who runs a small restaurant a few kilometres from the crash site, told AFP by telephone.
Fears for the fate of the jet spread on Monday afternoon as local media reported that China Eastern flight MU5735 had not arrived as planned in Guangzhou after taking off from Kunming shortly after 1:00pm (0500 GMT).
Flight tracking website FlightRadar24 showed no data for the flight after 2:22 pm.
The tracker showed the plane sharply dropped from an altitude of 29,100 feet to 3,225 feet (8,870 to 982 metres) in three minutes, before flight information ceased.
Al Jazeera’s Katrina Yu said it was the biggest aviation incident China has had in years.