Others said the tracks appeared to have been tampered with.

Maoist posters

Bhupinder Singh, the police chief in West Bengal, said a portion of the train track was missing, and that a Maoist group, the People's Committee Against Police Atrocities (PCPA), had claimed responsibility for the act of sabotage.

"We have recovered two posters by a local Maoist militia from the site of the accident," he said.

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"They have claimed responsibility for the incident in the posters. It is a clear case of sabotage. The Maoists have done it."

Al Jazeera's Prerna Suri, reporting from New Delhi, said no official confirmation has come from the federal government which "still maintains it was sabotage".

"However, these attacks follow a similar pattern but we will have to wait and see how seriously these links are being taken by the government," she said.

Reports from the crash site described a scene of chaos, with several passenger cars badly damaged and many dead and injured.

"The blast derailed 13 coaches of the Gyaneshwari Express. These coaches then fell on the other track where a goods train rammed into some of them," Soumitra Majumdar, a railway spokesman, told Reuters news agency, pointing to a suspected explosion as the cause of the crash.

"We fear many casualties. There could be many people dead. We don't have details yet."

Passengers trapped

Singh said many passengers were still trapped in the mangled coaches.

"It seems there are still a large number of passengers trapped in the carriages - dead or alive, we are not sure," he said.

Rescue workers with bolt cutters struggled to force their way into the most badly damaged coaches to try and free anyone still alive inside.

Paramedic teams treated the injured on the side of the track, while the most serious cases were taken away by air force helicopters.

Mamata Banerjee, India's railways minister, rushed to the scene of the collision early on Friday.

"The railways are a soft target. They are a lifeline ... which the Maoists have attacked in the past and, it seems, even now," she said.

Maoist fighters in the area have been blamed for a series of attacks on police, government buildings and infrastructure such as railway stations.

The rebels had called for a four-day strike in the area starting on Friday.

In recent months they have stepped up attacks in response to a government security offensive to clear them out of their jungle bases.

Earlier this month an attack on a bus in the mineral-rich state of Chhattisgarh killed 35 people.

The Maoist uprising began in West Bengal state in 1967 in the name of defending the rights of tribal groups, and has since spread to 20 of India's 28 states.

Manmohan Singh, India's prime minister, has described the uprising as India's biggest internal security challenge.