Taiwan's China Airlines said on Friday it is probing the opening of a cabin door on a Boeing 747-400 flight from Taipei to Los Angeles early this month. "We are investigating this incident, and we apologize to the passengers on that flight," Chen Peng-yu, head of CAL's press office, told reporters.

"When the plane took off from the Taoyuan International Airport, the crew made sure all the cabin doors were fastened, so we do not know why it opened in mid-air. We have asked the Boeing Company to explain it," he said.

The incident did not cause any damage to the Boeing jet, he added.

The incident occurred on December 8. CAL, Taiwan's largest airline, admitted it Friday morning after the Apple Daily had reported it, being tipped off by one of the passengers on that flight.

According to the passenger, identified as Mr Tan, he noticed something was wrong five minutest after Flight CI008 had left Taipei for Los Angeles at 11:52 pm on December 8.

Tan, who was sitting in the business class cabin, said he heard commotion and crying in the economy cabin, and learned that a cabin door in the back of the jet had opened.

The door that snapped open was on the left side of the Boeing jet, near row 46 and 47.

As wind gushed into the cabin, some passengers began to cry as crew members tried to close the cabin door. When that failed, the jet - which was flying at an attitude of 1,000 feet - dumped 6.3 tons of fuel over the sea and returned to the Taoyuan airport.

After ground checks and a delay of four hours, the plane with 264 passengers aboard took off again for Los Angeles, the Apple Daily said.

CAL wants Boeing to probe why the door opened in mid-air, and If the cabin door was not fastened when the plane took off, why the warning light had not gone off.

Aviation expert Lee Shih-ping said that the sudden opening of a cabin door is extremely dangerous for the aircraft and passengers.

"If the cabin suddenly loses pressure, passengers can suffer from hypothermia or pass out from loss of oxygen. The cabin door or even passengers could be sucked out of the plane," he said.