Los Angeles - The death toll from the worst train crash in southern California history mounted Saturday with at least 17 people dead and 135 injured.

As hundreds of rescue workers continued to sift through the mangled wreckage, officials warned grimly that the casualty figures were expected to rise.

The horrific accident occurred at 4:23 pm (2323 GMT Friday) when a freight train slammed into a packed commuter train on a stretch of single track near Chatsworth, in the San Fernando Valley north of Los Angeles.

Rescuers said victims were piled up inside three mangled passenger cars. During the night, the teams used hydraulic jacks to keep the passenger car from falling over and other specialized rescue equipment to gently tear apart the metal.

"There's so much damage, we need to wait for heavier equipment," Fire Captain Steve Ruda told reporters at the scene.

"This is the worst accident I've ever seen," Los Angeles Mayor Anthony Villaraigosa said. "Clearly the injuries are going to mount and so are the fatalities."

The commuter train was busy at the time, with an estimated 350 to 400 passengers. The Metrolink commuter train had just left the Chatsworth station and was on a sharp bend when the crash occurred.

According to the operators, two drivers were manning the Union Pacific freight train bound for Los Angeles.

The causes of the accident were not immediately known. The Los Angeles Times quoted a retired engineer who had worked the route as saying that the passenger train should have waited on a siding for the freight train to pass. The trains were both using the same stretch of track, because of the narrow tunnels which run through the region's mountains.

Television footage of the site of the accident showed fire department personnel rescuing injured passengers from an overturned wagon. Debris from the freight train was strewn across the tracks.

Rescue helicopters flew the injured to nearby hospitals, and scores of ambulances drove to the site of the crash.

About 250 fire department personnel and 200 police officers were on the scene Saturday. (dpa)