Federal Aviation Administration spokeswoman Laura Brown said Asiana Airlines 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 Star 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.
National Transportation Safety Board chairman Deborah A.P. Hersman and other NTSB officials are to board a plane from Washington, D.C. to San Francisco to investigate today's crash of a South Korean airliner at San Francisco International Airport, the NTSB reported.
Hersman and a "go-team" from the NTSB were set to get on a flight at Ronald Reagan Washington National Airport after holding a news conference at about 2:30 p.m. Pacific Time, according to NTSB spokesman Keith Holloway.
San Francisco General Hospital is treating 10 people -- including two children -- who were injured in the Asiana Airlines crash at San Francisco International Airport this afternoon, a hospital spokeswoman said.
All of the patients are in critical condition, hospital spokeswoman Rachael Kagan said.
She said there are six male patients and four female patients, and that the adults range in age from their 20s to their 40s.
The hospital is expecting additional patients, and is setting up tents outside the emergency room to accommodate walk-in patients and others whose injuries might not involve trauma-level care, Kagan said.
Facebook COO Sandberg changes flight
Facebook COO Sheryl Sandberg just posted to her Facebook page that she was supposed to be on the Boeing 777 that crash landed in San Francisco today along with some of her family and colleagues.
She posted on her Facebook page, "We switched to United so we could use miles for my family's tickets."
Samsung executive survives crash
Samsung executive David Eun was on Flight 214 and escaped without injury. He used Twitter throughout the ordeal to update what was going on inside the plane and at the airport.
Two runways reopened around 3:30 p.m., according to a tweet from SFO.
The Associated Press, ABC7 News, and Bay City News contributed to this report.