Firefighters and rescue teams are at the scene of the downed Asiana Airlines Flight 214, which had taken off from South Korea's capital, Seoul.
|Eyewitness Ki Siadatan: "[The plane] seemed like it was out of control"|
Some 60 people remain unaccounted for.
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.
San Francisco Fire Chief Joanne Hayes-Whites said 48 people were initially taken to nearby hospitals.
She said 190 people walked to safety from the plane, some with minor injuries - 82 of whom were later transferred for medical treatment.
While the sequence of events remains unclear, it appeared the plane landed and then crashed on San Francisco International Airport's Runway 28L, 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..."
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.