At least eight people were killed and dozens more injured when a tour bus crashed in southern California.

The bus driver, who survived the crash, reported to authorities that the bus experienced a mechanical failure of some kind at the time of the accident.

California Highway Patrol Sergeant Billy Rangel said he could not elaborate, but local media accounts cited unconfirmed reports of issues with the brakes.

The crash occurred last night on Highway 38 near the town of Yucaipa, about 113km east of Los Angeles,

Sgt Rangel said there was no immediate indication that alcohol or drugs or road conditions were factors in the crash.

The chartered bus was carrying a group of people home from a ski outing in California's Big Bear Lake resort area back to Tijuana, Mexico, when it struck a car from behind on the way down the mountain and careened out of control.

The bus then rolled over on its side and slid into a pickup truck that was heading uphill in the opposite direction, throwing several passengers from the bus.

Sgt Rangel said eight people were confirmed killed and 35 others were taken to hospitals with injuries, one of them in critical condition.

The drivers of the car and pickup truck struck by the bus also survived, he said.

He said authorities were still trying to determine the precise number of people who were on the bus at the time.

The bus was operated by Scapadas Magicas, a tour company based in National City, Southern California.

The highway is one of three roads leading to Big Bear Lake, a mountainous recreational area popular with skiers.