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 Boeing 737 flight from Kunming city to the southern hub of Guangzhou "lost airborne contact over Wuzhou" city in the Guangxi region on Monday afternoon, according to the Civil Aviation Administration of China (CAAC).

"It has been confirmed that this flight has crashed," the CAAC said, adding that it had activated its emergency response, while hundreds of firefighters were dispatched to the scene.

The plane was carrying 123 passengers and nine crew members, the CAAC said. 

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 p.m. local time.

Flight tracking website FlightRadar24 showed no data for the flight after 2:22 p.m.

The tracker showed the plane had sharply dropped from an altitude of 29,100 feet to 3,225 feet in the span of three minutes before flight information stopped.

State broadcaster CCTV said the crash "caused a mountain fire" that was later extinguished, as rescuers poured to the scene in Teng county near Wuzhou. 

One villager told a local news site that the plane involved in the crash had "completely fallen apart" and he had seen nearby forest areas destroyed by a fire caused when the plane crashed onto the mountainside.

China Eastern changed its website to black and white colors on Monday afternoon and opened an emergency assistance phone number.

Debris

President Xi called for "all efforts" toward the rescue and to find out the "cause of the accident as soon as possible", according to state broadcaster CCTV.

"We are shocked to learn of the China Eastern U5735 accident," he said.

A villager surnamed Liu told state-run China News Service that he had driven a motorbike to the scene after hearing a loud explosion.

He said he saw debris on the ground, including an aircraft wing and fragments of clothing hanging from trees.

China had enjoyed an enviable air safety record in recent years as the country was crisscrossed by newly built airports and serviced by new airlines established to match breakneck growth over the last few decades.

A Henan Airlines flight crashed in northeastern Heilongjiang province in 2010, killing at least 42 out of 92 people on board, although the final toll was never confirmed. It was the last Chinese commercial passenger flight crash that caused civilian casualties. 

The deadliest Chinese commercial flight crash was a China Northwest Airlines crash in 1994, which killed all 160 onboard.