SAN FRANCISCO (AFP) - Asiana Airlines Flight 214 was flying at an air speed of just 85 knots right before it crashed at San Francisco airport, well below the recommended landing speed of 137 knots for the Boeing 777, according to independent tracking data.

Two Chinese schoolgirls were killed on Saturday and more than 180 others injured when the Asiana flight from Seoul clipped a seawall short of the runway and went skidding out of control on its belly, shredding the tail end of the plane and starting a fire.

Ms Deborah Hersman, the head of the National Transportation Safety Board (NTSB), which is heading the probe into the accident, said Sunday that the plane was travelling well below the recommended landing speed.

"We are not talking about a few knots here or there. We're talking about a significant amount of speed below 137," Ms Hersman said.

She declined to provide a precise figure.

But according to flight logs published by Flightaware.com, Flight 214 had an air speed of 123 knots when it was at 300 feet, an airspeed of 109 knots when it was at 100 feet and an airspeed of 85 knots when it was at 200 feet - when the pilot apparently tried to climb again and abort the landing.

The request to abort was captured on the cockpit voice recorder 1.5 seconds before the plane crashed, according to Ms Hersman.

The flight data recorder also showed that as the Boeing 777 approached the runway, its pilots were warned that the aircraft was likely to stall and asked to abort the landing.

According to Asiana, the pilot at the controls at the time, Mr Lee Kang Kuk, had 43 hours of experience in piloting the 777 and was still undergoing training, although he had more than 9,000 hours of total flight time experience.

Asiana Flight 214 originated in Shanghai and stopped in Seoul and had 291 passengers and 16 crew members on board at the time of the accident.