At least seven people were killed and 75 others injured when two subway trains collided during the evening rush hour in Washington DC.

Mayor Adrian Fenty called the crash the deadliest in the 33-year history of the city's Metro subway system.

Officials said both trains were travelling on the same line when the incident happened - one was stationary and the other piled into it from behind. But the cause of the crash was not immediately clear.

At least one car from one of the trains ended up on top of the other in the incident, which occurred on tracks above the ground.

Authorities said one of those killed was a female train operator in the trailing train. The accident trapped passengers in one or more of the subway cars.

Emergency workers propped ladders up against train doors to help people escape from the mangled carriages.

President Barack Obama and his wife sent condolences: "Michelle and I were saddened by the terrible accident in Northeast Washington DC, today. Our thoughts and prayers go out to the families and friends affected by this tragedy."

Federal investigators will lead an inquiry into the incident, which occurred on the busy red line between Fort Totten and Takoma stations on the northeastern outskirts of the city. Both trains were heading south into the city.

It was the first crash involving a passenger death since 1982, when three people were killed in a derailment. The Metro train system began service in 1976.

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