At least 20 people have been killed and several others injured in a collision between a train and a passenger bus in southwest Pakistan.

The collision occurred in Kandhra town of Sukkur district, located some 500 kilometres (311 miles) from the port city of Karachi, on Friday.

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Sukkur Commissioner Shafiq Mahesar confirmed 20 deaths to reporters, fearing a rise in the death toll as the condition of several of those injured appeared to be critical.

Dozens of other passengers were also injured in the accident, the latest in a string of similar incidents in recent years.

"It is a huge tragedy and all administration and police officials are rushing to the site," district commissioner Shafique Ahmed Mahesar told dawn.com.

"It was an unmanned railway crossing as nothing existed there to prevent any traffic," he added.

Live images broadcast by local tv channel Dawn News showed rescuers retrieving bodies and injured people from the badly damaged bus and shifting them to the ambulances.

Pakistan has a long history of train accidents mainly because of poor infrastructure and lack of safety standards.

At least 23 people were killed and more than 72 others injured when a passenger train collided with a stationary freight train near Pakistan's central town of Rahim Yar Khan last year.