Asiana Airlines Flight 214 was flying at an airspeed of just 106 knots when it crashed at San Francisco airport, well below the recommended landing speed of 137 knots for the Boeing 777, the head of the US National Transportation Safety Board (NTSB) said Monday.

Two Chinese schoolgirls were killed on Saturday and more than 180 other people were injured when the 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.

Deborah Hersman, the chairwoman of the NTSB, which is heading the probe into the accident, told a press conference that the plane's flight data recorder showed the plane was traveling approximately 106 knots at impact.

She said that was "significantly slower than the target approach speed of 137 knots."

"137 knots is the speed that they want to have when they cross the threshold of the runway," Hersman said.

She said the plane's airspeed was around 149 knots when it was at an altitude of 1,000 feet -- 54 seconds prior to impact.

The speed then fell to 134 knots at 500 feet, 34 seconds prior to impact, and to 118 knots at 200 feet, 16 seconds before it hit the ground, she said.

At 125 feet, eight seconds prior to impact, the throttles started moving forward, Hersman said, as the pilot apparently attempted to abort the landing and perform what is known as a go-around. The airspeed at the time was 112 knots.

She said that three seconds prior to impact the flight data recorder showed the lowest recorded airspeed -- 103 knots.

The website Flightaware.com had reported earlier, citing flight logs, that Flight 214's airspeed had dropped as low as 85 knots during the landing run.

According to Asiana, the pilot at the controls at the time, 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 aboard at the time of the accident. (AFP)