Image copyright Reuters Image caption Air China is one of the country's main carriers

A co-pilot smoking an e-cigarette on an Air China flight caused the plane to start a rapid emergency descent, investigators have said.

They say he tried to hide the fact that he was smoking but accidentally shut off the air-conditioning, causing oxygen levels to fall.

The crew on Tuesday's flight from Hong Kong to the city of Dalian released oxygen masks and brought the plane more than 6,500m (21,000ft) lower.

It later returned to cruising altitude.

An initial probe by China's Civil Aviation Administration in China has shown that the co-pilot tried to turn off a fan to stop smoke reaching the passenger cabin without telling the captain, but turned off the air-conditioning unit instead.

Passengers say they were told to fasten their seat belts as the plane had to descend.

Image copyright Weibo Image caption People posted images online of the dropped oxygen masks on the flight

The regulator's safety officer Qiao Yibin said the crew had to perform emergency measures, dropping oxygen masks until they could figure out the problem.

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

Once they saw that the air conditioning had been turned off, they reactivated it and brought the flight back to its normal altitude.

Authorities are reportedly investigating the cause "in greater detail", examining both the flight data recorder and the cockpit voice recorder to determine precisely what caused the incident.

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The airline promised a "zero-tolerance" approach to crew misbehaviour on Chinese social media site Weibo.

Chinese flight regulations prohibit all flight crew from smoking, and banned passengers from using e-cigarettes on board in 2006.

But there have been accusations of pilots smoking on board other Chinese flights, including in 2015 when the state-run radio spoke to passengers on a Hong Kong-Beijing flight who claimed to smell strong smoke coming from the cockpit.