SAN FRANCISCO — An Asiana Airlines flight crashed as it was landing at San Francisco airport on Saturday, a federal aviation official said. It wasn't immediately known if there were any injuries. Federal Aviation Administration spokeswoman Lynn Lunsford said the Boeing 777 crashed at San Francisco International Airport while landing. It wasn't immediately clear what caused the crash. A video clip posted to Youtube showed smoke coming from a silver-colored jet on the tarmac. Passengers could be seen jumping down the inflatable emergency slides. The airline is based in Seoul, South Korea.