The bus fell a couple hundred feet before stopping, Oregon State Police said on its website, adding that the chartered bus was reportedly heading home to Vancouver, Canada from Las Vegas, Nevada.
Police in the northwestern state are still investigating the cause of the crash, which occurred in the morning near the city of Pendleton, in northeastern Oregon.
The rescue effort was complicated by the drop-off from the highway, and police said “personnel trained for rope rescues” were called in to help.
About 40 passengers appeared to have been onboard the bus, but the numbers have not yet been confirmed.
The names and nationalities of the victims and other passengers were not immediately released, but many “are believed to be out of country residents,” according to police.
Although the bus driver survived the crash, investigators have not been able to interview him due to the severity of the injuries.
There was at least one other fatal crash in northeastern Oregon early Sunday due to snowy and icy conditions, police said, urging all drivers to take precautions while traveling.