WASHINGTON (AFP) - The pilots of a South Korean airliner that crashed in San Francisco last year depended too much on automated systems they didn't understand, the head of the National Transportation Safety Board (NTSB) said Tuesday.

Acting NTSB chairman Christopher Hart made the statement at the start of a day-long hearing to establish the probable cause of the Asiana crash that left three dead and 187 injured.

"The Boeing 777 is one of the more sophisticated and automated aircraft in service," said Mr Hart in his opening remarks.

"But the more complex automation becomes, the more challenging it is to ensure that pilots adequately understand it," he said.

"In this instance, the flight crew over-relied on automated systems that they did not fully understand. As a result, they flew the aircraft too slow and collided with the seawall at the end of the runway."

The crash of Asiana Flight 214 on July 6 was a the first fatal commercial airline disaster in the United States since 2009.

The Boeing 777 was completing an otherwise routine 10-1/2 hour flight from Seoul when it clipped the seawall at San Francisco International Airport July 6 with its landing gear, skidded off the runway and burst into flames.

All three of the fatalities were young Chinese women, including one who was struck by a fire truck beneath a wing covered with firefighting foam.

Mr Hart said the NTSB's aim was "to help prevent similar accidents in the future," before on-the-ground investigators took turns presenting their findings.