At least 28 people have died after a fire swept through a moving train bound for the southern Indian city of Chennai, local and interior ministry officials say.

The fire was discovered at a railway station in Nellore, in Andhra Pradesh state early on Monday, B Sreedhar, the local administration chief, said.

The blaze has been extinguished and at least 22 injured people have been hospitalised. The coach was completely gutted, with rescuers forced to use gas cutters to access parts of it.

Sreedhar said the fire was believed to have been caused by a short circuit in the coach, though a railways ministry spokesman said it may have been caused by "someone carrying inflammable materials on the train".

After the fire was discovered, Sreedhar said, the stricken section was detached from the rest of the train to prevent the blaze from spreading.

Al Jazeera's Prerna Suri, reporting from New Delhi, said that survivors had shared some "horrifying" accounts of the fire.

"They say that since this accident happened at 4:30am local time (23:00 GMT on Sunday), most passengers were asleep, and so when the fire broke out, most did not have much time to escape. Some are saying that the doors were also jammed, literally trapping a few people inside," she reported. 

The fire on the New Delhi-Chennai train was reported at Nellore, a town nearly 500km south of Hyderabad in Andhra Pradesh state.

Sreedhar said that identifying the dead was a difficult task.

"Some of the bodies are charred beyond recognition,'' he said, adding that officials had made preliminary identifications based on the reservations chart from the train's records.

Images of the site showed thick black smoke still pouring out of the charred carriage, as dawn broke over the accident scene. Dozens of rescuers, survivors and crowds of onlookers milled around the burned carriage, as the blackened bodies of victims were pulled out of the wreckage and laid in rows alongside the railway line.

Family members of the victims wailed and screamed, while other dazed survivors sat quietly with their belongings.

"I woke up when people were rushing into our compartment, I was in S-10 which was attached to the S-11 coach that caught fire," passenger Shantanu, who gave only one name, told the NDTV news channel. "There was smoke all around. We tried to open the emergency window, people jumped out of it."

The carriage was designed to carry around 70 people, and was travelling at about 110km per hour when it passed through Nellore station.

Railway Minister Mukul Roy said an investigation was underway.

"Nothing can be excluded and nothing can be said without an investigation," Roy told reporters shortly before rescue officials wrapped up their nearly 12-hour search for bodies.

Accident-prone network

India's rail network is still the main form of long-distance travel in the huge country, despite fierce competition from private airlines.

In May, there were two fatal accidents on the rail network, including a collision that killed 25 people near the southern city of Bangalore.

Four passengers died in a separate accident after a train derailed in the northern state of Uttar Pradesh.

In March, then Railways Minister Dinesh Trivedi unveiled a draft budget for 2012-13 that includes a major safety upgrade, to be financed by across-the-board fare increases.

He was, however, forced to withdraw it and resign after his own political party, the Trinamool Congress, objected to the ticket price increase.

Prime Minister Manmohan Singh sent his condolences to the victims following the accident on Monday, and has asked the national railways ministry to coordinate the relief effort, his office said.

India's worst rail accident was in 1981, when a train plunged into a river in the eastern state of Bihar, killing an estimated 800 people.