Recovery teams have found human remains at the crash site of the China Eastern jet which plunged into mountains this week, say state media.

Authorities have yet to declare the number of victims, but there has been no sign so far that any of the 132 people onboard had survived.

Investigators still do not know why the plane suddenly entered a near-vertical dive in southern China on Monday.

But the recovery of the intact cockpit voice recorder is hoped to yield clues.

The black box was found damaged on the outside but its internal records appeared to be fine, officials said. It has been sent to Beijing for its data to be analysed.

Search teams found the first of the crucial black boxes in difficult circumstances on Wednesday.

Heavy rains have flooded the steep and rugged landscape of the crash site in Wuzhou, making efforts harder for hundreds of rescuers and volunteers, who have so far also found scattered plane debris and the charred remains of personal belongings.

On Wednesday, dozens of family members of those onboard the flight began to arrive at the scene. Escorted by officials and supporters, many kept their heads bowed as they passed gathered media but some spoke briefly to journalists about their loss.

One man told Reuters his nephew had been a passenger on the flight. He and his son had driven five hours from neighbouring Shenzhen province in the hopes of being able to find their relative alive.

"I plan to stay until the matter is sorted and I can take his ashes back. But this depends on the work of authorities.

"I hope the country can thoroughly investigate this matter and find out whether it is the manufacturer's fault or a maintenance problem."

The crash of the passenger plane this week - which had been on a short domestic flight from Kunming to Guangzhou on the coast- has shocked and devastated China. The country has had a strong air safety record over the past three decades.

President Xi Jinping swiftly ordered a full-scale investigation into the tragedy, dispatching Vice Premier Liu He and hundreds of personnel to the rural hillside region in Guangxi province.

But none of the knowledge released so far about the flight indicates any technical problems or anything amiss with environmental factors.

At a press conference on Wednesday, aviation officials said there hadn't been any weather or other hazards endured by the flight on its route path. China Eastern said the Boeing 737-800 plane, less than seven years old, had also passed all pre-flight checks.

There had been three pilots, China Eastern officials disclosed. The captain had 6,709 hours flying experience, and the first and second officers had 31,769 hours and 556 hours respectively.

"From what we know, the performance of the three pilots had been good and their family life relatively harmonious," the airline representative said.

Air traffic control had also been in regular communication with the pilots on board right until it entered its near nose-dive descent from a cruising height.

Flight tracking data showed the plane plummeted thousands of metres in minutes.

According to FlightRadar24, the plane was cruising at an altitude of 29,100ft (about 9,000m), but two minutes and 15 seconds later it was recorded at 9,075ft. The last sourced information on the flight showed it ended at 14:22 local time, at an altitude of 3,225ft.

Investigators are expected to look at several possible causes, including deliberate action, pilot error, or technical issues such as a structural failure or mid-air collision.

China investigators have also invited US aviation experts to join in the investigation - as the Boeing plane was manufactured in the US. The invite has been welcomed by the Biden administration.

But US officials said they weren't sure if they would send over a team yet due to China's strict Covid quarantine requirements for incoming visitors.

In the meantime, China's aviation officials are carrying out a two-week safety audit of all planes while China Eastern and its two subsidiaries have grounded its fleet of Boeing 737-800s as an emergency precaution.