At least 10 people were killed and more than 100 injured on Friday, with those figures expected to rise, after a freight train and commuter train collided head-on outside Los Angeles.

Some four hours after the trains, which were headed in opposite directions on the same track, slammed together west of Los Angeles, officials feared that more bodies could still be trapped under the twisted wreckage.

"There are at least 10 fatalities, confirmed by the coroner's office. That number will likely grow because as you know we are still in the rescue phase," Los Angeles Mayor Antonio Villaraigosa told reporters.

"There are still people that haven't been extricated," Villaraigosa said.

The mayor said officials were having a difficult time tallying the number of people injured in the worst commuter train crash in Los Angeles history but that it included dozens and was "probably over 100."

The Los Angeles Times said on its website that the death toll would likely rise to at least 15 and that at least one police officer was among the dead.

A city-wide "tactical alert" was called across Los Angeles as rescuers worked into the night to pry the twisted metal apart and reach victims still trapped.

They said it was possible more bodies would be found when the train cars could be moved with heavy equipment.

The commuter train held more than 350 passengers and crew. It was not immediately clear why the two trains were on the same track.

"I was riding, sitting down, minding my own business when all of a sudden - boom, people go flying all over the place," 67-year-old passenger Willie Castro told the Times.

"Everyone started screaming. You could hear that everyone was in pain," he said in a story on the newspaper's website.

The commuter train left downtown Los Angeles' Union Station at about 3.30 p.m. The train could carry between 350 and 400 people and officials said it was likely almost full.

Police trying to identify the dead, wounded and missing asked family members to come forward. A spokeswoman for Metrolink said the cause of the crash would be investigated.