Two express trains have collided in southern Pakistan, killing at least 38 passengers, authorities said.

Up to 20 passengers are still trapped in the wreckage of the Millat Express train, said Umar Tufail, a police chief in Ghotki district in Sindh province, where the collision occurred before dawn.

Authorities are trying to arrange heavy machinery to rescue those still trapped, he added.

The Millat Express derailed and the Sir Syed Express train hit it soon afterwards, said Usman Abdullah, a deputy commissioner in Ghotki.

It was not immediately clear what caused the derailment and the subsequent collision.

“Right now the challenge for us is to quickly rescue those passengers who are still trapped in the wreckage,” he said.

Azam Swati, minister for railways, told the Associated Press that so far 35 people had died and dozens of others were injured.

“I am on my way to the Ghotki district where the tragic train accident happened today,” he said.

Mr Swati said engineers and experts were trying to determine what caused the collision.

He said all aspects will be examined, including the possibility of sabotage.

“We are not sure about the cause of the train accident at this stage,” he said.

According to railway officials, about 1,100 passengers were on board the two trains, and arrangements were being made to assist the survivors.

Trains after a collision in Ghotki, Pakistan
The trains after the collision (AP)

A military statement said that troops were participating in the relief and rescue at the scene.

It said military doctors and ambulances were also dispatched from a nearby city and a team of military engineers was sent to Ghotki by helicopter.

According to local media, some of the passengers were travelling on the Millat Express train to attend a wedding party but it was unclear whether they were among the dead or injured.

TV footage showed ambulances transporting injured passengers to hospitals.

According to Pakistani TV stations, heavy machinery had not reached the scene about four hours after the crash.

Pakistan’s prime minister expressed his deep sorrow over the tragedy.

Imran Khan said on Twitter that he had asked the railway minister to supervise the rescue work and also ordered a probe into the crash.

This image from a video shows a train after a collision in Ghotki, Pakistan
One of the trains after the collision (AP)

Officials at Pakistan railways said they had ordered an investigation and rescue work was still in progress.

Aijaz Ahmed, the driver of the train that collided with the derailed train, told Pakistan’s Geo News TV that on seeing the derailed train cars, he tried his best to avoid the accident by braking but failed.

He did not explain how he survived.

Officials said at least 50 passengers had been transported to hospital, where some of the passengers were listed in a critical condition.

Malik Aslam, a local villager, told Geo News TV that about 100 people were injured and he counted at least 30 bodies of passengers during the rescue and recovery work.

Train accidents are common in Pakistan, where successive governments have paid little attention to improving the poorly maintained signal system and ageing tracks.

In 1990, a packed passenger ploughed into a standing freight train in southern Pakistan, killing 210 people in the worst rail disaster in Pakistan’s history.