A black box from the crashed China Eastern airliner was recovered on Wednesday as investigators try to determine what made the jet carrying 132 passengers nosedive into a mountainside in southern China.

The cause of the disaster has mystified aviation authorities who have scoured rugged terrain for clues, finding no survivors from what is almost certain to be China's deadliest plane crash in nearly 30 years.

Aviation officials confirmed they had found a black box they believed to be the cockpit voice recorder, which should provide important clues to the cause of the crash.

Zhu Tao, an official at China's aviation authority, told reporters that the exterior was "severely damaged, and the storage unit was also damaged to a certain extent, but it was relatively complete".

The stricken jet, a Boeing 737-800, was equipped with two recorders: one in the rear passenger cabin tracking flight data, and the other a cockpit voice recorder.

Rescuers are still looking for the other recorder.

Officials have not declared all of the passengers dead despite the pulverized mass of twisted metal and charred belongings that recovery teams found at the crash site.

Zheng Xi, head of the area's rescue service, said Wednesday evening that some human remains had been found.

Earlier in the day AFP reporters saw a small crowd of people guided by officials across the police cordon that marks entry to the site, huddled under umbrellas in the driving rain.

One middle-aged man later confirmed that he was the relative of someone on the flight, and asked the media not to crowd around him.

Rescue work is ongoing across the huge 45,200 square meter crash zone.
Orange-clad rescuers have been using dogs, drones and scanners to scour the debris scattered across the mountainside.

- Sharp drop -

The Boeing 737-800 plane went down near Wuzhou in southern China on Monday afternoon after losing contact with air traffic control.

Up until that point, communication with ground staff had been normal, state media reported.

Tracking website FlightRadar24 showed the jet sharply dropped from an altitude of 29,100 to 7,850 feet (about 8,900 to 2,400 meters) in just over a minute.

After a brief upswing, it dropped again to 3,225 feet, the tracker said.

The captain had more than 6,700 hours of flight experience and the first co-pilot had more than 31,000 hours of flight time, officials said on Wednesday.

There was a second co-pilot on board, with more than 550 hours of flight time, and all three were in good health with no known personal problems.

Rescuers were forced to pause the search on Wednesday as rain raised risks to teams working in the zone, where a large pit had been bored out by the impact of the aircraft.

A reporter for state broadcaster CCTV given access to the crash area said there were risks of "small-scale landslides" as rain had destabilized the steep slopes.

The rescue service chief told reporters that water had built up in the crater at the center of the accident site, which was being pumped away.

Authorities have sealed off access to the crash site and blocked foreign media from speaking to the distraught relatives who have gathered in Wuzhou.

President Xi Jinping was swift to order a probe into the crash, dispatching senior Communist Party officials to the scene -- including close aide Vice Premier Liu He.

The Civil Aviation Administration of China (CAAC) said it would conduct a two-week safety inspection across the industry.

China Eastern said the crashed plane, which was nearly seven years old, had met all airworthiness requirements pre-flight.

Aviation authorities said more painstaking evidence gathering was needed before coming to any conclusions.