The head of the US National Transportation Safety Board has said that the pilots aboard the Asiana Airlines Boeing 777 that crashed in San Francisco relied on automatic equipment.

The auto-throttle system maintains airspeed and the pilots did not realise the plane was flying too slowly until it was just 60 meters above the ground.

NTSB Chairwoman Deborah Hersman also said two flight attendants were ejected from the plane after its tail hit a seawall in front of the runway and was torn off.

Both were found injured but alive on the side of the runway.

Ms Hersman said many questions remained about the incident.

The South Korean airline's flight crew members were not tested for drugs or alcohol after the crash, a requirement for pilots of US-based carriers involved in accidents, she said.

The accounts given to investigators by the pilots, showed the plane was travelling 25% below its target airspeed as it came in for landing.

While she has declined to speculate on the cause of the crash, much of the information released by the NTSB suggests pilot error as a main focus of the investigation.

The pilot in charge of landing the plane on Saturday was in training on the 777 and was roughly halfway through the process, while seated next to him was a co-pilot on his first flight as an instructor.

Both were experienced pilots, although they had not flown together before, Hersman said.

Referring to the instructor pilot, she said it was not until 200 feet that "he recognised the auto-throttles were not maintaining speed" and tried to abort the landing.

Ms Hersman had previously said that the plane had been at an altitude of 200 feet 16 seconds before crashing.

Three of the four pilots on board were in the cockpit during the landing, although only two could see the runway.

Ms Hersman said an examination of the wreckage showed that the auto-throttle was "armed," but it was not clear if it had been properly engaged or had somehow failed before the plane slowed to a near-stall and hit the ground.

She noted that the pilots were responsible for maintaining airspeed.

The world's largest pilots union rebuked the NTSB for its handling of the crash investigation.

The union said the agency had released too much information too quickly, which could lead to wrong conclusions and compromise safety.

Ms Hersman rejected the criticism. "We work for the traveling public," she said. "We feel it is important to show our work."

Ms Hersman did not comment on whether anyone in addition to the two flight attendants was ejected from the plane, though the two teenage Chinese students who died were found outside the aircraft.

Asiana Airlines Chief Executive Yoon Young-doo arrived in San Francisco on Tuesday to meet with US investigators, Asiana staff and survivors of the crash.

Ms Hersman also confirmed witness accounts that at least one emergency escape chute had deployed inside the aircraft, trapping a flight attendant.

The pilot who was sitting in the cabin worked to free her, Ms Hersman said.

She was eventually freed and hospitalised with serious injuries.