By Eric M. Johnson

(Reuters) - Two mothers and their four children were killed early on Tuesday after their minivan was struck from behind by a tractor-trailer and burst into flames as it tumbled down an embankment north of Los Angeles, the California Highway Patrol said.

After surviving the crash, the fathers of the victims suffered burns to their arms trying to free their children and wives trapped inside the van, said California Highway Patrol spokesman Frank Romero.

"When we arrived on scene, we saw the van starting to catch fire. We saw the husbands come running up to us, told us their families were in the vehicle still," Officer Dan Williams told Los Angeles television station KABC in video posted online.

Williams said in the video interview that he and his partner tried in vain to extricate the women and children after the crash, which occurred along Interstate 5 about 60 miles north of Los Angeles.

"My partner went and tried to get in through the van door," Williams said. "The flames came at him, he had to back out. I grabbed the fire extinguisher, I tried to extinguish the flames, but the van burst into flames very, very quickly and we were unable to assist in getting anybody out of the van."

Los Angeles County Fire Department Inspector Gustavo Medina said by phone that the department dispatched units at about 3:30 a.m. local time to the crash site near the community of Gorman.

Romero, the California Highway Patrol spokesman, said the cause of the crash was still being investigated.

The husbands, identified as Aaron Hon Wing Ng, 34, of San Francisco, and Wei Xiong Li, 45, of Daly City, were airlifted to a hospital and expected to live, Romero said. The names of the other victims were not yet released. The ages of the children were also not released.

The van initially came to rest on the far right shoulder of the highway after a collision with another car. But the rear end of the van remained in the traffic lane and was then struck from behind by the big rig, Romero said.

The driver of the truck, Richard Lopez, 60, of Walnut, California, was uninjured and not arrested.

Footage of the crash site showed the burned out van angled backward down a steep embankment off the side of the highway.

(Reporting by Eric M. Johnson in Seattle; editing by Sharon Bernstein and Dan Grebler)