The driver of one express misread a signal and ploughed into a stationary train full of sleeping passengers near the remote town of Ghotki, then a third train slammed head-on into the wreckage, according to railway officials.

Rescuers were trying to save dozens of people still imprisoned in the mangled carriages of the three trains, which lay scattered amid piles of debris and body parts.

"It's a painful scene. There are bodies scattered all over. People are crying, fathers are looking for children, husbands for their wives and brothers for their sisters," a witness told AFP by telephone.

Pakistan's President Pervez Musharraf ordered an urgent investigation and promised to punish those responsible for the pre-dawn pile-up, the country's worst in 15 years.

"It's a tragic accident, I am deeply saddened I express my condolences," he told state television. "It was not sabotage, it appears to have been caused by some carelessness."

Musharraf said 107 people were confirmed dead but Chaudhry Nazir Ahmed, divisional superintendent of Pakistan Railways, told AFP that about 150 had been killed.

Local hospitals reported that 132 bodies had been brought in so far and rescue workers continued to pull more from the carriages, officials said.

Around 1,000 people were injured, local police chief Agha Mohammad Tahir said, and many of the victims were women and children.

The accident happened when one of the trains, the Quetta Express, stopped for repairs at Sarhad station near Ghotki and the Karachi Express coming from Lahore smashed into it around 4:00 am (2300 GMT).

A number of carriages were catapulted onto a parallel track and a third train, the Tez Gam Express heading for Islamabad's twin city Rawalpindi, then careered into them.

Railway officials blamed the Karachi Express driver for misreading a signal at Sarhad station that turned green to allow the Quetta Express to move off.

The train was travelling at a speed of 120 kilometers (75 miles) an hour when it hit and the driver and his assistant were both killed, Junaid Qureshi, a senior railway official, told AFP.

"The driver of the Karachi Express thought the signal allowed him to pass and he rammed into the rear part of the Quetta Express," Qureshi said.

Most of the casualties were said to be on board the Tez Gam train. In all 16 coaches were derailed -- 12 from the Tez Gam train, three from the Quetta Express and one from the Karachi Express, along with its engine.

Rescue workers used electric cutters to recover bodies and wounded passengers from the smashed carriages, while two relief trains have also been rushed to the area to help with rescue efforts, officials said.

"Our teams have rescued scores of people from inside the ruptured coaches, but hundreds more are trapped inside and we are trying to evacuate them," Salahuddin Haider, spokesman for the government of Sindh province, told AFP.

Troops have cordoned off the area while around 50 ambulances from civil and military hospitals and the private Edhi Welfare Trust were shuttling between hospitals and the disaster site, witnesses said.

The smash halted all railway traffic in the area, which is 400 kilometres northeast of Karachi, and it would take many hours to restore the service, Qureshi said.

Military spokesman Major General Shaukat Sultan said the army was supervising the rescue after the "tragic accident", while civilian services and relief agencies were also taking part.

Most of the injured were rushed to Ghotki's civil hospital and army and paramilitary doctors were drafted in from a nearby garrison to help, Sultan said.

Worried relatives thronged to several railway stations across Pakistan to inquire about family members travelling on the trains,

Dozens of people have been killed in recent years on Pakistan's ageing railway system.

In 1991 another crash at Ghotki between a passenger train and a goods train killed 50 people according to authorities and between 100 and 200 according to press reports.

A year earlier, more than 350 people were killed and 700 injured when a goods train collided with a passenger train in Sangi, near Ghotki.