A passenger train derailed and overturned late on Tuesday in Philadelphia, killing at least six people and injuring dozens of others.

Emergency personnel said some of the injured were critical after the accident on the train heading from Washington to New York. Others walked away from the crash with light injuries.

The train's seven cars, including the engine car, were crushed, turned over on their side or upside down in the late evening disaster. One was unrecognizable as a train car.

As night fell, rescuers with flashlights scoured through the wreckages.

"It is an absolute disastrous mess," Philadelphia Mayor Michael Nutter told reporters. "I have never seen anything like this in my life."

Officials declined to speculate on the cause of the incident, though some experts suggested the crash may have been due to a track defect or wheel failure.

Witnesses said the front of Amtrak Train 188 shook as it went into a turn, and the six cars behind it then went off the rails.

An estimated 243 people, including five crew members, were aboard the train when it crashed around 9.30 pm (local time).

Nutter warned that the casualty and injury estimates were only preliminary. He also would not confirm whether authorities had accounted for all those aboard the train.

Hydraulic tools had to be used to remove passengers from some of the most badly damaged train cars, firefighters said.

"I've never seen anything so devastating," said Fire Commissioner Derrick Sawyer. "You can see they completely derailed from the track, destroyed completely and they've been overturned completely."

Former US Congressman Patrick Murphy of Pennsylvania, who was on the train, said he was sitting on a bench in the cafe car when the train began to topple.

"It went to my right, then to my left. Everyone who was on the left side of the car, where I was sitting, just got thrown completely over to the right side," he said.

Murphy tweeted a photo from inside a wrecked train car. He said he was uninjured and trying to assist other passengers.

Murphy said the train seemed to be going 60-70 miles per hour (around 100-110 kilometers per hour) when it suddenly derailed and rolled. Passengers had to kick out a window to escape.

Some of the injured were unable to move, Murphy said.

Another passenger, Jeremy Wladis, 51, was in the last car of the train when he felt the jolt. He said he saw "phones, laptops, everything flying", The Philadelphia Inquirer newspaper reported.

"There were women launched up in the luggage rack," he said. "I don't even know how they got there."

Hundreds of emergency personnel, including firefighters and police, were deployed.

(China Daily 05/14/2015 page12)