About 600 soldiers, firefighters and police marched to the crash site, a patch of about 1 sq km in a location hemmed in by mountains on three sides, after excavators cleared a path, state television said.
It added that the search for the recorders, or "black boxes", of the Boeing 737-800 involved in China's first crash of a commercial jetliner since 2010, would be carried out in grid-by-grid fashion, probably through the night.
Flight MU5735 was headed on Monday for the port city of Guangzhou from Kunming, capital of the southwestern province of Yunnan, when it plunged from cruising altitude to crash in the mountains of Guangxi less than an hour before landing time.
A jet appeared to dive to the ground at an angle of about 35 degrees from the vertical in video images from a vehicle's dashboard camera, according to Chinese media.Â
Si, 64, a villager near the crash site who declined to give his first name, told Reuters he heard a "bang, bang" at the time of the crash.
"It was like thunder," he said.
State media called the situation grim, saying the possibility of the deaths of all aboard could not be ruled out.
State television has shown images of plane debris strewn among trees charred by fire. Burnt remains of identity cards and wallets were also seen.
Police set up a checkpoint at Lu village, on the approach to the site, and barred journalists from entering. Several people gathered for a small Buddhist ceremony nearby to pray for the victims.
The last commercial jetliner to crash in China was in 2010, when an Embraer E-190 regional jet flown by Henan Airlines went down, killing 44 of the 96 aboard.
Highlighting the top-level concern, Vice Premier Liu He went to Guangxi on Monday night to oversee search and rescue operations. An official of the same rank was similarly sent to the site of the 2010 crash in northeast China.
The disaster comes as planemaker Boeing seeks to rebound from several crises, notably the impact of the coronavirus pandemic on air travel and safety concerns over its 737 MAX model following two deadly crashes.
Once it is found, the cockpit voice recorder could yield clues to what went wrong with Monday's flight.
"Accidents that start at cruise altitude are usually caused by weather, deliberate sabotage, or pilot error," Dan Elwell, a former head of US regulator the Federal Aviation Administration, told Reuters.
Elwell, who led the FAA during the 737-MAX crisis, said mechanical failures in modern commercial jets were rare at cruise altitude.
The US National Transportation Safety Board (NTSB) on Monday appointed an investigator, as the Boeing aircraft was produced in the United States, but it was unclear if the investigator would travel to China.
On Monday, China Eastern and two subsidiaries grounded its fleet of 737-800 planes. The group has 225 of the aircraft, data from British aviation consultancy IBA shows.
As of Tuesday, other Chinese airlines had yet to cancel any flights that use 737-800 aircraft, according to data from Chinese aviation data provider Flight Master.
