The manoeuvre had not been requested by air traffic control and the reason why the plane did this is yet to be explained.
However, the recording shows that the air traffic controller had asked another plane in the area to stay at a higher altitude and one possible explanation is that the Air China pilot had misheard or misunderstood the instruction sent to the other plane.
In the incident, which happened over Tuva, a remote part of Siberia that borders Mongolia, the two planes came within 300-400 feet (around 90-120 metres) of each other, much closer than the global minimum standard of 1,000 feet, according to live tracker Flightradar24.
Radar shows that the near-collision happened when Air China Flight CA967, an Airbus A350 travelling from Shanghai to Milan, started climbing from a height of 34,100 feet to 36,000 feet between 21.39 and 21.52 GMT on Sunday, July 6 (just before 6am the following day Chinese time).
