An Asiana Airlines Boeing 777 jet with more than 300 people on board clipped a seawall short of the runway at San Francisco International Airport on Saturday, snapping off its tail and bursting into flames.
Media reports said at least two people who were aboard Flight 214 from Seoul were killed. At least 10 others, including two children, were in critical condition, one of the city's hospitals said.
Video footage showed the jet on its belly surrounded by firefighters with debris scattered on the runway and in the surrounding area.
There was no official explanation for the crash landing but images appeared to indicate the aircraft struck a rocky area at the water's edge some distance from the runway. Pictures showed the tail detached from the fuselage and the landing gear had also sheared off.
US Federal Aviation Administration spokesman Lynn Lunsfor confirmed the crash landing.
"It looked normal at first... the wheels were down," an unidentified man who witnessed the crash told CNN. "It just hit (the seawall) like that and the whole thing just collapsed immediately.
"It just pancaked immediately. The wings caught on the tarmac."
A National Transportation Safety Board team was heading to San Francisco to investigate the crash landing.
"Everything is on the table at this point," NTSB chairwoman Debbie Hersman told reporters in the US capital when asked if pilot error was to blame.
"We have to gather the facts before we reach any conclusions," she added.
One dramatic photo tweeted by someone claiming to be a survivor showed people streaming out of the jet. An inflatable slide was at the front entrance. Other emergency exits also appeared to have been used.
The airport was closed after the incident but two runways later reopened. Some flights were diverted to Los Angeles.
"I just crash landed at SFO. Tail ripped off. Most everyone seems fine. I'm ok," one survivor, David Eun, wrote on Twitter.
But another photo from above showed a more distressing scene, with most of the roof of the plane missing and the cabin seating area charred by fire. The aircraft's wings were still attached.
One passenger, Chun Ki-Wan, told YTN TV in Seoul by phone: "Most of the passengers escaped unhurt, following directions from the crew.
"I saw some passengers bleeding and being loaded onto an ambulance," Chun said.
"Everything seemed to be normal before it crash-landed."
Survivors have been ushered into a room at the airport guarded by police and airline personnel. Relatives were asked to wait a short distance away.
The plane left the South Korean capital on Friday and there were 291 passengers and 16 crew aboard, a spokesman for the airline confirmed.
Among those on board were 77 Koreans, 141 Chinese, 61 US citizens, and one Japanese national, the airline said in a statement.
The Asiana spokesman told AFP: "We're still checking what has caused the accident and how many casualties have occurred."
San Francisco General Hospital said it was treating six female and four male victims, including two children, all of whom were in critical condition. The adult casualties were aged between 20 and 40.
Local media cited multiple witnesses who said the plane had approached the runway at an awkward angle, with several onlookers saying they then heard a loud bang.
Emergency crews were reporting passengers in need of burn treatment, according to Redwood City Fire Department.
Anthony Castorani, who saw the flight land from a nearby hotel, said he saw the plane touch the ground -- and then noticed a larger plume of white smoke.
"You heard a pop and you immediately saw a large, brief fireball that came from underneath the aircraft," he told CNN.
The accident site was covered in white foam used by firefighters, with at least six fire trucks at the scene.
California highway police temporarily closed all roads near the facility -- a major international hub, especially for flights to and from Asia.
Asiana Airlines is based in Seoul. The twin-engine 777 aircraft is one of the world's most popular long-distance planes, often used for flights of 12 hours or more, from one continent to another.