A Boeing 777 aircraft has crash-landed at San Francisco international airport, killing at least two people and injuring dozens more, officials say.
Pictures posted on Twitter showed passengers jumping down the inflatable emergency slides and leaving the area, as plumes of smoke rise from the plane.
Fire fighters and rescue teams are at the scene of the downed Asiana Airlines Flight 214, which had taken off from South Korea's capital, Seoul.
The cause of the crash is unclear.
Early indications suggest the plane came in too short and hit the seawall at the airport.
Eight adults and two children who suffered critical injuries are being treated at San Francisco General Hospital, hospital spokesperson Rachael Kagan said.
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.
Rescue teams initially took 49 people deemed to be in a serious condition to nearby hospitals, officials said.
Some 190 people walked to safety from the plane, many of whom were later treated for minor injuries.
Most of the remaining passengers were waiting in the airport terminals uninjured. One person remained unaccounted for.
While the sequence of events remains unclear, it appeared the plane landed and then crashed on San Francisco International Airport's Runway 28 L, said Federal Aviation Administration spokeswoman Laura Brown.
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 appeared to have broken away from the main wreckage. ' Out of control '
Passenger David Eun tweeted a picture of people jumping out of 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..."
City Mayor Ed Lee:" This is still a fluid and active scene"
Mr Eun, who describes himself as a" digital media guy" and" frequent flier", added:" Fire and rescue people all over the place. They're evacuating the injured. Haven't felt this way since 9/11."
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."
He saw events unfold from the balcony of his home in the Millbrae area of San Francisco, which overlooks the airport.
Weather conditions were fine and there was little wind, he added.
Arrivals and departures at the airport have been suspended since the incident.
The twin-engine Boeing 777 has a good safety record as a long-haul aircraft and is used by many major carriers.
Asiana is South Korea's second-largest airline.
【新闻快讯】一架韩国韩亚航空公司的波音 777客机 6日在美国旧金山机场降落时坠毁，目前已经确认该机上有 141名中国人， 77名韩国人，以及 61名美国人。据外媒最新报道，目前该客机坠毁已导致 2人死亡， 80多人受伤。
韩亚航空是韩国的一家主要航空公司，成立于 1988年 2月，原名“首尔航空”( Seoul Airlines)，员工约 9600人，目前包括货运航线在内的通航目的地有百余个，航点以亚洲城市为主，同时也有数条欧美航线。
根据技术资料，波音 777型客机设计载客量可超过 300多人，航空公司一般载客两三百人。资料显示，波音 777型客机近年来没有遭遇过造成大规模人员伤亡的事故。
客机机体在事故中已严重损毁。美国国家运输安全委员会调查组已在事故现场收集物证，就事故原因展开进一步调查。
