At least 21 people, including children, were killed and dozens more injured when an express train derailed in northern India on Friday, authorities said.

Television images showed two mangled carriages lying beside the track after the accident, which occurred around 30 miles (50 kilometres) from Lucknow, capital of the northern state of Uttar Pradesh.

"The latest death toll is 21 and over 50 are injured," the chief public relations officer for the Northern Railway, Neeraj Sharma, told AFP. "Rescue work is still going on."

Earlier, authorities said two children had been killed in the crash.

Sharma said there would be an investigation into the cause of the accident, which could have been down to driver negligence or the brake system failing.

The Press Trust of India (PTI) news agency said the train apparently overshot a signal before coming off the rails.

An official at the scene said one of the coaches was vacant at the time of the accident, limiting the number of deaths.

Ambulances carrying teams of doctors rushed to the site, where rescuers were using metal cutters to try to free passengers trapped in the wreckage, PTI said.

Deadly accidents are common on India's railways, whose vast and rundown network carries tens of millions of people daily.

Last month a passenger train derailed in southern India after a boulder fell on the tracks, killing at least 10 people.