A crash investigation has begun into what caused an Amtrak train to derail in north Philadelphia, killing at least five people and leaving six others critically injured.

A sixth person was declared dead by Herb Cushing, the medical director of Temple University hospital in Philadelphia, where several of the injured were taken, according to local television news reports. During the night the mayor of Philadelphia, Michael Nutter, had confirmed that five people had died in the crash.

Federal investigators arrived on the scene of the mangled and overturned cars on Wednesday morning to try to establish the cause of the crash.

The Northeast Regional 188 service was carrying 243 people, including five crew, when it went off the rails between Washington DC and New York City shortly after 9pm local time. The front of the train was going into a long curve when it started to shake, according to passengers.

The carriages lay several yards from the rails in the dark in a twisted chain of wreckage, as passengers desperately struggled to get out.

Nutter described the scene as a "disastrous mess" and said some of the passengers had not been accounted for.