Investigators looking into the China Eastern Airlines crash face a “very high level” of difficulty due to the severe damage to the aircraft, an official has said.

Authorities do not have a clear assessment of the cause of Monday’s crash, said Zhu Tao, director of aviation safety at the Civil Aviation Administration of China.

He said no survivors had been found so far.

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“The jet was seriously damaged during the crash, and investigations will face a very high level of difficulty,” Mr Zhu said.

“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.

Flight 5735, a Boeing 737-800 carrying 132 passengers and crew, was travelling from Kunming to Guangzhou when it plunged from cruising altitude and crashed into mountains in Guangxi, igniting a huge fire that could be seen on NASA satellite images.

Footage showed the jet appearing to dive to the ground at a near-vertical angle and photos from the scene show debris from the plane strewn among burnt trees.

Around 600 soldiers, firefighters and police have been scouring the heavily forested mountain slopes in the area in their search for the plane’s black boxes, which hold the flight data and cockpit voice recorders.

Poor access to crash site

The crash site is surrounded on three sides by mountains and is accessible only by foot and motorcycle on a steep dirt road.

Rain fell on Tuesday afternoon as excavators dug a path to make access easier, state broadcaster CCTV said, with the steepness of the slope making the positioning of heavy equipment difficult.

Boeing Chief Executive Dave Calhoun has said the company was deeply saddened by the news and had offered the full support of its technical experts to assist in the investigation.

Plane entered steep dive

The plane was around an hour into its flight at an altitude of 29,000ft when it entered a steep, fast dive around 2.20pm local time, according to FlightRadar24.com.

It plunged 7,400ft before briefly regaining around 1,200ft in altitude, then began diving again.

It stopped transmitting data 96 seconds after starting to dive.

Asia correspondent

This is going to be a long and difficult investigation.

Even getting near the crash site is difficult – the only way Sky News could get to the operations centre was by hitching a ride on two volunteers’ motorcycles, skirting the rough roads beyond the police checkpoints.

It is ringed by mountains on three sides, with one path out.

And the proof of the violence of this crash is in just how little wreckage there is in images from state media. At other crash sites, you see chunks of fuselage and engines. Here there are only fragments.

Those tiny pieces are a terrifying end but they are also clues. They will have to be painstakingly put together to understand what happened, and the flight recorder and cockpit voice recorder must still be recovered.

Because right now it is a mystery: a modern jet, cruising comfortably, then plummeting to earth in minutes.

Captain Mike Vivia, former head of flight operations for the UK’s Civil Aviation Authority, cautioned against speculation but told Sky News that “the sort of areas investigators will be looking at will be sabotage, hijack, or suicide, along with the structural break up of the engine at the tail, because it’s the tail that controls the aircraft up or down”.

On Monday, China Eastern and two subsidiaries grounded its fleet of 737-800 planes.

According to British aviation consultancy IBA the group has 225 such aircraft.

It was China’s first fatal air crash since 2010, when 44 of 96 people died when an Ambraer E-190 regional jet flown by Henan Airlines crashed on approach to Yichun airport in low visibility.

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