A Boeing 777 aircraft that crash-landed at San Francisco airport killing two people did not have mechanical problems, an airline official has said.
The head of the South Korean airline Asiana, Yoon Young-doo, did not rule out human error but said the pilots were experienced veterans.
Most of the 307 people on board were injured, 49 of them seriously.
The plane came down short of the runway, ripping off its tail, after apparently hitting a sea wall.
Smoke billows from the burning plane after it crash-landed on San Francisco International Airport's Runway 28L
One survivor said the plane came in to land too fast and too low, but there was no warning of problems.
Passengers and crew escaped down emergency slides as it burst into flames.
Good record
Mr Yoon apologised "deeply" for the effect the accident had had on all those involved, bowing in front of TV cameras at a Seoul news conference.
He said there was no emergency alarm and the crew had made the usual requests to passengers to fasten their seatbelts to prepare for landing.
"Currently we understand that there were no engine or mechanical problems," he said.
The pilots were veterans, he added, and one had more than 10,000 flying hours.
Asiana confirmed that two female Chinese teenagers died in the crash. They had been seated at the back of the aircraft.
They are believed to be the first-ever fatalities in a Boeing 777 crash.
The twin-engine aircraft has a good safety record for long-haul and is used by many major carriers.
The only previous notable crash occurred when a British Airways plane landed short of the runway at London's Heathrow Airport in 2008.
More than 300 people were on Asiana Airlines Flight 214, which had taken off from South Korea's capital, Seoul
Firefighters and rescue teams were quickly at the scene
The tail fin broke off during the crash and could be seen lying on its side apart from the main wreckage
Fire engines could be seen spraying a white fire retardant into gaping holes in the wrecked plane's roof
Boeing said in a statement it would provide technical assistance to the investigation.
Five people are in critical condition at San Francisco General Hospital, hospital spokesperson Rachael Kagan said. Three others are being treated at Stanford Hospital.
Altogether 181 people were taken to hospital, mostly with minor injuries.
There were 291 passengers and 16 crew on board, Asiana said.
Nationalities on board included 141 Chinese, 77 South Koreans and 61 US citizens, the airline said.
All of the passengers have been accounted for.
Footage of the scene showed debris strewn on the runway and smoke pouring from the jet, as fire crews sprayed a white fire retardant into gaping holes in the craft's roof.
One engine and the tail fin were broken away from the main wreckage.
Quick evacuation
Passenger Ben Levy said there had been no warning of problems, although the plane appeared to be coming in too fast and too low.
"It happened in a flash, nobody was worried about anything," he said.
But once the aircraft crashed, "there was chaos, disbelief, screaming".
"My seat had been pushed to the floor, it was a mess everywhere," Mr Levy recalled.
Nevertheless, people "calmed down pretty quickly" and evacuated the plane without pushing or stepping on each other.
Meanwhile another passenger, David Eun, tweeted a picture of people evacuating down the plane's emergency inflatable slides and wrote: "I just crash landed at SFO. Tail ripped off. Most everyone seems fine. I'm ok. Surreal..."
A witness to the crash, Ki Siadatan, said the plane "looked out of control" as it descended over San Francisco Bay to land just before 11:30 (18:30 GMT).
"We heard a 'boom' and saw the plane disappear into a cloud of dust and smoke," he told the BBC. "There was then a second explosion."
Arrivals and departures at the airport have been suspended since the incident.
