A Pakistani express train with up to 600 passengers aboard derailed Sunday, killing at least three people and injuring as many as 40.

The diesel train went partly off the tracks around 7:30 p.m. throwing several cars onto their sides and sending one tumbling 100 yards to the bottom of a gorge in a rural area of the eastern province of Punjab.

Interior Minister Aftab Khan Sherpao said the cause of the derailment was under investigation.

An intelligence official on the scene told The Associated Press that the track was damaged before the train passed, causing the accident. The official, who spoke on condition of anonymity because he was not authorized to talk to the press, would not say how the track was damaged.

Pakistan army spokesman Gen. Shaukat Sultan told The Associated Press he had received confirmation of at least three deaths and up to 40 injuries. He said there did not appear to be more people trapped in the overturned cars.

Pakistan's Geo television reported that four passengers were killed and as many as 50 wounded. A rescue worker at the scene with Pakistan International Human Rights, Asif Mahood, also said four people had died.

Between 500 and 600 passengers may have been in the 10-car train when it derailed near the town of Domeli, about 60 miles southeast of Islamabad, said Salimur Rahman Akhund, general manager of operations with the state-run Pakistan Railways.

The train was probably traveling around 37 mph at the time of the derailment — a normal speed for that location — said Arshad Khattak, the railway operator's chief engineer.

One survivor, Javed Butt, 30, said he was in the first car when he heard a bang, followed by a strong jolt. Butt's car remained upright and he was able to walk away from the accident.

"I heard screams and I'm sure there are lots of people injured, women and children," he said, adding he could not see the wreckage because of the dark.