An emergency descent by an Air China aircraft after cabin oxygen levels dropped has been linked to a co-pilot smoking an e-cigarette during the flight, state media said on Friday.

The state-backed Air China Boeing 737 aircraft was flying to the Chinese city of Dalian from Hong Kong when it went down to 10,000 feet (3,048 m), with oxygen masks deployed. Then it climbed again to continue to its destination, an incident that fuelled the concerns of safety experts.

Chinese airlines have a good safety record in general, but passengers have, on occasion, accused pilots of smoking during flights. Few such incidents have been confirmed, however.

"In the preliminary investigation, the co-pilot was found to be smoking an e-cigarette," state-owned China News said, citing a news conference by the Civil Aviation Administration of China (CAAC) investigating Tuesday's incident.

"Smoke diffused into the passenger cabin and relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen," it quoted Qiao Yibin, an official of the regulator's aviation safety office, as saying.

China News added that the co-pilot had shut off the air conditioning units.

Qiao said the shut-off triggered an alarm, prompting the crew to peform an emergency pressure relief procedure, which then released the cabin's oxygen masks.

The crew realised the problem after the descent and restored the air conditioning, allowing cabin pressure to return to normal, he added.