* Plane crashed in heavily forested southern mountains on Monday
* China Eastern grounds all its Boeing 737-800 jets after crash
* Chinese media show video footage of plane's near vertical dive (Updates with mild edits)
By Martin Quin Pollard
WUZHOU, China, March 22 (Reuters) - Rescuers in southern China searched for victims from a China Eastern Airlines jet on Tuesday after it crashed with 132 people on board and authorities said severe damage to the aircraft would make it difficult to establish the cause of the crash.
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. Reuters could not immediately verify the footage.
No survivors have been found, said Zhu Tao, director of aviation safety at the Civil Aviation Administration of China (CAAC), told reporters.
"The jet was seriously damaged during the crash, and investigations will face a very high level of difficulty," Zhu said at the first government briefing on the disaster.
"Given the information currently available, we still do not have a clear assessment of the cause for the crash," he said, adding that the aircraft did not respond to repeated calls from air controllers during its rapid descent.
On Tuesday, rescuers combed heavily forested mountain slopes in southern China, using shovels and torches in their search for victims and flight recorders from the jet.
About 600 soldiers, firefighters and police marched to the crash site, a patch of about 1 square kilometre (0.4 square mile) 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.
'BANG, BANG'
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 television has shown images of plane debris strewn among trees charred by fire. Burnt remains of identity cards and wallets were also seen.
Rain was forecast in the area this week.
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.
"That B737-800 jet met airworthiness standards before taking off and technical conditions were stable," Sun Shiying, a China Eastern official, said at the briefing.
"The crew members were in good health, and their flying experience was in line with regulatory requirements," he said.
PLANES GROUNDED
The disaster comes as 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.
"Accidents that start at cruise altitude are usually caused by weather, deliberate sabotage, or pilot error," Dan Elwell, a former head of U.S. 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 737-800 was delivered on June 22, 2015, and accumulated 18,239 hours of flight time after 8,986 flights, said Zhu.
Chinese investigators are leading the probe because the crash took place there, but U.S. authorities will provide aid because the plane was U.S.-made.
The U.S. National Transportation Safety Board (NTSB) said on Tuesday it was in touch with CAAC's investigator-in-charge and would support the investigation with technical advisers from the Federal Aviation Administration, Boeing and engine manufacturer CFM in all ways necessary.
China has strict quarantine policies due to the pandemic. No decision has been made yet about whether U.S. authorities will travel there for the investigation, a NTSB spokesperson said.
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.
(Additional reporting by Stella Qiu, Albee Zhang and Ryan Woo in Beijing, Jason Xue and Brenda Goh in Shanghai, Jamie Freed in Sydney, David Shepardson in Washington, Allison Lampert in Montreal and Eric M Johnson in Seattle; Editing by Simon Cameron-Moore, Clarence Fernandez, Raissa Kasolowsky and Bernard Orr)
