An Air China copilot is accused of smoking an electric cigarette in the cockpit and mistakenly turning off the cabin air conditioner, causing the emergency descent of a flight from Hong Kong to the northeast city ofDalian, China's civil aviation regulator said on Friday.

Flight CA106 received an altitude alert when the aircraft was midair on Tuesday night. The crew released oxygen masks and made a rapid descent, according to the Civil Aviation Administration of China.

The copilot of the flight was attempting to turn off the circulating fan to prevent the smoke and smell of his e-cigarette from spreading to the cabin, but accidentally shut down the nearby switch for the air-conditioning unit, according to a primary investigation of the administration.

The mistake caused an insufficient oxygen supply to the cabin and triggered the altitude alert. Crew then released oxygen masks in the passenger cabin and made an emergency descent to 3,000 meters, in line with emergency procedures.

The crew later realized the mistake and turned the air conditioning back on. The administration noted that the captain was not informed when the copilot turned off the air conditioning.

"If a violation is confirmed, the copilot will be severely punished," Qiao Yibin, a senior official with the aviation safety office of the administration, told a press conference on Friday. The details of the incident are still under investigation.

The aircraft landed safely at Dalian airport at 10:31pm and all 153 passengers and nine crew members were safe.

Air China, the nation’s flagship carrier, has said in a statement that the company will take a no-tolerance approach if any violation is found among crew. “Any violators will be severely punished,” the airline promised on Thursday.