The device was severely damaged, and it was not immediately clear if it was the flight data recorder or the cockpit voice recorder, an official of the Civil Aviation Administration of China (CAAC) told a media briefing.

Flight MU5735 was en route from the southwestern city of Kunming to Guangzhou on the coast on Monday when the Boeing 737-800 jet suddenly plunged from cruising altitude at about the time when it should have started its descent ahead of landing.

Most of the jet appears to have disintegrated upon impact and no survivors have been found.

The cause of the crash has yet to be determined, with aviation authorities warning that their investigation would be very difficult because of the severe damage to the aircraft.

"We found a black box on the scene but this black box has been severely damaged on the outside, so the investigation team on the front-line is still ascertaining whether it is the flight data recorder or the cockpit voice recorder," Mao Yanfeng, head of aircraft investigation at CAAC, told the briefing.

Weather along the flight path on Monday did not pose any danger to the aircraft, and air controllers had maintained communication with it after its take-off and prior to its rapid descent, Mao told reporters.

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