By Subir Bhaumik BBC reporter in Calcutta

The Indian army has killed four separatist Bodo rebels in the north-eastern state of Assam.

Nearly 40 rebels, belonging to two of Assam's strongest separatist groups, have been killed this year while trying to enter the state from their bases in Bhutan.

Bhutan has been exerting pressure on the rebels to vacate their bases in the southern part of the kingdom and has threatened military operations against them unless they leave peacefully.

Indian troops encircled a hideout of the outlawed National Democratic Front of Bodoland (NDFB) near Tangla town in Assam's central district of Darrang early on Wednesday, police said.

Captain killed

When the troops called out to the rebels over loudspeakers and asked them to surrender, the rebels fired back.

Four NDFB rebels, including a self-styled captain, were killed and several others were injured.

But all those rebels who were injured managed to flee with the other guerrillas.

So far this year, more than 40 rebels from the NDFB and an Assamese separatist group, the United Liberation Front of Assam (ULFA), have died while trying to enter the state from bases in southern Bhutan.

Under pressure

Bodos make up 13% of Assam's population

Bhutan has been applying pressure on the rebels to leave the kingdom or face military action.

The Bhutanese Government itself is under pressure from India to take action to curb the activities of the rebels.

Major General Gaganjit Singh, who commands a mountain division on the India-Bhutan border, says his troops now have complete domination of the hill terrain and have covered all the tracks used by the rebels.

He said the military operation explained the high number of casualties in the rebel ranks.

However, he denied charges by the commander of ULFA's military wing, Paresh Barua, that the Indian troops often cross into Bhutanese territory to attack rebel squads.