An Asiana Airlines plane, Boeing 777, has crashed at San Francisco International Airport, Fox News reports:
More details are not yet available.
UPDATE: Here's another video:
UPDATE II: Someone using Twitter handle @Eunner claims to have been on the plane:
UPDATE III: The apparent passenger adds, "Lots of activity here. Friends, pls don't call right now. I'm fine. Most people are totally calm and trying to let the fire and rescue do their jobs. Just like during 9/11, most people are great and try to be helpful in crisis... — David Eun"
UPDATE IV: A "loud boom" was reportedly heard by witnesses: