At least 33 people have been killed and more than 120 injured after a train collision near the southern Pakistani town of Dharki, officials say, with rescue work still underway to find survivors trapped in the debris.

The collision took place early on Monday morning near the town of Dharki, about 440km (273 miles) north of Pakistan’s largest city of Karachi, officials say.

"The number of dead bodies has increased to 33, while the number of injured is near 120,” senior health official Dr Manzoor Jakhrani told Al Jazeera by telephone from one of the main government hospitals were the injured were being treated.

Rescue work is ongoing at the site and is in its "final stages”, he said.

Television footage from the site of the crash showed the mangled wreckage of the two trains strewn across the track, with rescuers using cranes and other equipment to try and reach those trapped inside.

The accident took place when eight compartments of the northbound Millat Express train derailed near Dharki, local official Razzaq Minhas told Al Jazeera. The southbound Sir Syed Express passenger train then hit the derailed compartments, he said.

"Rescuers are still trying to get into at least two crashed bogeys, they are still removing the debris,” he said. "Equipment and paramilitary forces are there to remove the debris.”

Pakistani Prime Minister Imran Khan said he was "shocked” by the crash, and ordered Railways Minister Azam Swati to reach the site of the accident on Monday.

"Have asked railway minister to reach site and ensure medical assistance to injured and support for families of the dead,” he tweeted. "Ordering comprehensive investigation into railway safety faultlines.”

The Pakistani military said it had dispatched rescue helicopters, ambulances and specialized search and rescue crews to the site of the crash.