At least nine people have died after a FedEx tractor-trailer crossed a grassy median strip on a California highway and crashed head-on into a tour bus carrying Los Angeles-area high school students.

Massive flames could be seen devouring both vehicles just after the crash, and clouds of smoke billowed into the sky until firefighters had quenched the fire, leaving behind scorched black hulks of metal. Bodies were draped in yellow plastic inside the burned-out bus.

The crash happened a little after 5.30pm local time on Interstate 5 near Orland, a small city about 160 kilometres north of Sacramento.

The bus was one of two that the admissions office at Humboldt State University in northern California had chartered to bring prospective students in the state to tour the Arcata campus, Humboldt’s vice president of administrative affairs Joyce Lopes said.

The drivers of the bus and truck were killed, officials said on Thursday. The other fatalities were passengers on the bus.

A California Highway Patrol (CHP) dispatcher said the bus and truck were on opposite sides of the freeway when the truck crossed the median and slammed into the bus, causing an explosion and fire.

Investigators say the truck driver might have been trying to avoid a passenger car that was also involved in the crash, which shut down north- and south-bound traffic on the freeway.

A first responder who helped set up a triage at the scene said 36 or 37 people received injuries ranging from severe to minor burns, broken legs and noses and head lacerations.

“The victims were teenage kids. A lot of them were freaked out. They were shocked. They still couldn’t grasp what happened,” Jason Wyman with the Orland Volunteer Fire Department said.

CHP officer Tracy Hoover said at least one person suffered from “severe burns” while others had “cuts, scrapes, minor burns and contusions”.

Witness Marc Smutny, 27, said he was doing landscape work nearby when he heard a series of blasts.

Smutny said he ran to the scene and saw flames rising from the FedEx truck and a white charter bus.

“It was insane. The bus was engulfed in flames, smoke in and out of the front,” Smutny said. “The bus looked like it took most of the hit…. It was horrible.”

Los Angeles Unified School District Superintendent John Deasy said some students from LA Unified schools were on board the chartered bus, but that the district had no word on any fatalities.

At least one LA Unified student was injured, he said. That student contacted her mother, who alerted district officials.

In a statement on the university’s website, Humboldt officials said the charter bus was bringing a group of prospective students to the campus’ April 12 Spring Preview Day.

“Our hearts go out to those who have been affected, and we are here to support them, and their families, in any way possible,” said Rollin Richmond, the university’s president, in a statement.

Late on Thursday, California Governor Jerry Brown issued a statement about the accident, saying that he and his wife, Anne, “extend our heartfelt and deep sympathies to the families, friends and loved ones of those who died”.