At least 22 people were killed and more than 40 injured when two passenger trains collided Thursday morning near Karachi's Landi Railway Station, officials said.

Divisional Superintendent (DS) Railway Nasir Nazeer said up to 60 people could be injured in the accident, adding that there could have been up to 1,000 passengers in both trains.

Officials said rescuers armed with metal-cutting equipment and heavy cranes had managed to pull all the passengers from the twisted wreckage. "No one is left inside," Ijaz Ahmad Khan, a Karachi administrative official, told reporters at the scene.

Adviser to Sindh Chief Minister on Labour Senator Saeed Ghani said that railway officials gave a green signal to the train coming from behind by mistake, and it went on to hit the train at rest.

Nazeer said that an inquiry has been launched into the cause of the accident.

Witnesses described watching in horror as Zakaria Express from Multan rammed into Fareed Express from Lahore, which was parked there, with the roar of the crash swiftly followed by the screams of people trapped inside.

At least two carriages from the trains overturned due to the collision. TV footage showed the trains had suffered heavy damage.

The train operation came to a complete halt after the accident.

Drivers ignored signals: Saad Rafique

Federal Minister for Railways Khawaja Saad Rafique pointed to the negligence of the driver and assistant driver of the train coming from behind as possible cause for the incident.

He said the drivers were first shown a yellow signal — an indication to slow down — followed by a red signal, "the SOP of which is stop [the train] dead for a minute and then proceed very slowly".

He said the drivers ignored both signals, which "apparently" led to the collision.

Both the driver and assistant driver of the train are missing, the minister said, adding that a search is on find them. An initial report into the incident will be presented in the next 72 hours, he said.

Rafique said one track of the accident site has been restored, while the restoration of the other will take some time.

"Financial assistance can never be a replacement for human life," he said, while announcing a compensation of Rs1.5 million for the family of each killed and Rs350,000 for the injured.

He said "negligence of some irresponsible officials" appeared to be cause of the incident because the railway's signalling system was working "totally fine".

The minister earlier said an "independent inquiry" will be held to hold accountable those responsible for the incident.

'Please help me, I am dying'

Many people were rushed to Jinnah Hospital, where an AFP reporter described horrifying scenes as the injured lay screaming and crying while medics rushed to help them.

"I am dying, I am dying, please, please, I am dying," cried Abdul Ghaffar, 55, as doctors tried to move his legs and hands.

He appeared to have multiple injuries, while his children and wife were also wounded and lay on beds nearby.

Other victims appeared too stunned to talk. Many had head and foot injuries, and at least one man had his leg amputated below the knee.

'Huge' crash

Factory worker Ajab Gul told AFP he was on his way to work when the accident occurred.

“Suddenly another train came speeding in and smashed into the parked train,” he said, describing the sound of the crash as “huge”.

“There were clouds of dust and smog. After that we heard screams. People inside the collided trains were screaming and crying.”

Onlookers rushed to their aid, he said, adding that he had helped pull 17 people, including women and children, from the wreckage.

Train accidents are common in Pakistan, which inherited thousands of kilometres of track and trains from former colonial power, Britain.

The railways have seen decades of decline due to corruption, mismanagement and lack of investment.

Earlier in September, four people were killed as a passenger train collided with a goods train near Multan.

Last November, 19 people were killed in Baluchistan after a train's brakes failed and it sped down the side of a mountain.

In July 2015, at least 17 people were killed when a special military train fell into a canal after a bridge partially collapsed.