At least 12 people have been killed and 50 injured after two bombs exploded on a passenger train in the eastern Indian state of West Bengal, police say.

The blasts ripped apart two carriages of the Haldibari passenger train near Belakova village, in north-west Bengal.

Police fear some passengers could still be trapped in the wreckage.

It is not yet clear who was behind the blasts. Police say they suspect the involvement of separatist groups active in West Bengal and Assam.

Critically injured

Police say eight people died on the spot and four later succumbed to injuries in hospital.

"The casualties could rise as some passengers still appear to be trapped in the wreckage of the train," a senior police official told the BBC.

Railway officials say the carriages affected were supposed to join up with the Darjeeling Mail that links northern Bengal with the state capital, Calcutta.

"The blast took place just before the joining of the compartments," officials said.

Fire and thick smoke has hampered rescue efforts.

The injured passengers are being taken to hospital in the town of Jalpaiguri - while the critically injured are being taken to the North Bengal Medical College in Siliguri.

The BBC's Subir Bhaumik in Calcutta says this is the first major explosion in the region since 1999, when a blast hit a military train carrying Indian soldiers to the Kargil front in Kashmir.

Ten soldiers were killed in that attack, which the authorities blamed on a radical Islamic group.