Federal Aviation Administration spokeswoman Laura Brown said Flight 214 crashed while landing on runway 28 left at 11:26 PDT.
A video clip posted to YouTube showed smoke coming from a jet on the tarmac. Passengers could be seen jumping down the emergency slides.
Television footage showed the top of the fuselage was burned away and the entire tail was gone. One engine appeared to have broken away. Pieces of the tail were strewn about the runway. Emergency responders could be seen walking inside the burned-out wreckage.
Stephanie Turner saw the plane going down and the rescue slides deploy, but returned to her hotel room before seeing any passengers get off the jet, she told ABC News. Turner said when she first saw the flight she noticed right away that the angle of its approach seemed strange.
"It didn't manage to straighten out before hitting the runway," she said. "So the tail of the plane hit the runway, and it cartwheeled and spun and the tail broke off ... I mean we were sure that we had just seen a lot of people die. It was awful.
"And it looked like the plane had completely broken apart," she said. "There were flames and smoke just billowing."
A call to the airline seeking comment wasn't immediately returned.
The National Transportation Safety Board said it was sending a team of investigators to San Francisco to probe the crash. NTSB spokeswoman Kelly Nantel said Saturday that NTSB Chairman Deborah Hersman would head the team.
Asiana is a South Korean airline, second in size to national carrier Korean Air. It has recently tried to expand its presence in the United States, and joined the oneWorld alliance, anchored by American Airlines and British Airways.
The 777-200 is a long-range plane from Boeing. The twin-engine 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. The airline's website says its 777s can carry between 246 to 300 passengers.
The flight was 10 hours and 23 minutes, according to FlightAware, a flight tracking service. The aircraft is configured to seat 295 passengers, it said.
The last time a large U.S. airline lost a plane in a fatal crash was an American Airlines Airbus A300 taking off from John F. Kennedy International Airport in New York in 2001.
Smaller airlines have had crashes since then. The last fatal U.S. crash was a Continental Express flight operated by Colgan Air, which crashed into a house near Buffalo, N.Y. on Feb. 12, 2009. The crash killed all 49 people on board and one man in a house.
Associated Press writers Joan Lowy in Washington, D.C., and Scott Mayerowitz in New York contributed to this report.