The train was on its way from Haldibari to New Jalpaiguri in the state when the explosion hit one of its carriages at Belakoba station, 500km from the state capital Kolkata, said Prasad Ranjan Roy, West Bengal's home secretary.

"Six people were killed and 60 wounded in the blast," Trikal Rabha, a spokesman for Northeastern Frontier Railway, told AFP from Guwahati city in neighbouring Assam state.

"The toll may go up as 25 people are in a critical state."

Rescue workers have reached the spot and are shifting the injured to hospitals in New Jalpaiguri, 18 kilometres (11 miles) from site of the blast, Rabha added.

It was not immediately known what caused the explosion as the area where the blast took place is in a remote corner of West Bengal state, bordering the Himalayan countries of Bhutan and Nepal, police said.

Separatist rebels of the Kamatapur Liberation Organisation (KLO) -- fighting for a separate homeland -- are known to be active in the region, police said.

The KLO is known to maintain close links with the banned United Liberation Front of Asom, one of the most powerful insurgent groups operating in India's restive northeast.