Image copyright Reuters Image caption File picture of another Air China aircraft

Chinese authorities are investigating why a plane was forced to drop more than 6,500m (21,000ft) due to loss of cabin pressure.

The Air China flight from Hong Kong to Dalian lost altitude rapidly and oxygen masks dropped in the cabin.

The plane then returned to cruising altitude and continued as scheduled.

There is speculation in local media that crew had been smoking in the cockpit and then mishandled the ventilation system.

Chinese aviation authorities are examining both the flight data recorder and the cockpit voice recorder to determine what caused the incident.

Several passengers on board the Tuesday flight posted pictures of the dropped oxygen masks on social media.

Image copyright Weibo Image caption Despite the incident, people remained calm

According to some people on board, cited on Chinese media, passengers were told to fasten their seat belts and told that the plane had to descend.

If a plane loses cabin pressure, the pilot has to bring the aircraft to a lower altitude to keep crew and passengers safe.

"These planes fly at an altitude that at Mount Everest is called the death zone because of a lack of oxygen," aviation expert Greg Waldron of Flight Global told the BBC.

"So the pilot absolutely has to bring the plane lower. Otherwise, crew and passengers would get unconscious."