By Subir Bhaumik BBC correspondent in Calcutta

Three policemen including a senior official have died after police forces from two north-east Indian states mistakenly fought a gunbattle.

Two of the dead are from the Tripura state police, and one was from the state police in neighbouring Meghalaya state.

Officials in Tripura said their police had received information that a group of separatist insurgents from the neighbouring state of Manipur were travelling in a bus coming into Tripura from Assam.

They took up positions at a checkpoint near the state's northern town of Dharmanagar and challenged the armed men in civilian clothes in the bus once it arrived.

The armed men opened fire and the encounter lasted for nearly three hours.

Later, it was discovered that the armed men in the bus were not insurgents - but policemen from the neighbouring state of Meghalaya who had come to rescue a trader kidnapped from their state earlier in the week.

When the Meghalaya police were asked why they had not informed the Tripura police or disclosed their identity when challenged, they did not respond.

Covert operations

Tripura police deputy superintendent Kalyan Sarkar and his bodyguard died during the encounter, while a sub-inspector of Meghalaya police was also killed.

Separatists are often targeted in covert operations

Three Tripura policemen and two Meghalaya policemen were injured, all hit by bullets.

In India's troubled north-east, police forces often resort to secret operations against insurgents and rarely inform their counterparts in other states.

The police in Assam have conducted many such operations in neighbouring states, even killing senior separatist leaders in such actions.

But this is the first time that a secret operation has ended up with a clash between two police forces.