"Clearly the injuries are going to mount and so are the fatalities."
Arnold Schwarzenegger, the governor of California, said "human failure" was believed to be behind the disaster and said the death toll is expected to rise further.
'So much damage'
Steve Ruda, a fire captain, said rescue workers were cutting through the top deck of the double-decker train to reach passenger spaces.
"There's so much damage. We need to wait for heavier equipment," he said.
Dr Stephanie Hall, the chief medical officer at the Los Angeles County-USC Medical Centre, said two females and a male were in critical condition.
Rescue crews put out a fire caused by the collision and worked to free trapped survivors as heavy equipment was brought in to move the carriages.
Julio Pedraza, a witness, said that the crash "made a terrible sound, like a bomb, a huge noise".
Denise Tyrrell, a spokeswoman for the Metrolink rail system, said that the train had left Union Station in Los Angeles and was travelling northwest to Moorpark when the collision happened.
She said that there would normally be about 350 people on board the train in rush hour.
"We don't know if we hit another train or another train hit us," Tyrrell said.
The incident comes three years after 11 people were killed and 180 others injured when their train derailed and struck another after hitting a sports utility vehicle that had been doused in petrol and left on rail tracks in the Los Angeles suburb of Glendale.
Labourer Juan Alvarez, 29, was jailed for life for first-degree murder in June in connection with the crash.