By Subir Bhaumik BBC correspondent in Calcutta

Four policemen have been killed in an attack by separatist rebels in India's north-eastern state of Manipur.

Officials said the rebels ambushed the convoy of a senior police superintendent near the New Heaven village on the road connecting the state's capital, Imphal, with the town of Ukhrul.

The four policemen, they said, were killed in the gunbattle that followed.

This is the second major rebel attack on policemen in just several days in the frontier state, which is located on India's border with Burma.

Earlier this month, nine personnel of the central reserve police force had died in an attack near Manipur's only hydro-electric project on the Loktak Lake.

Fight for independence

It was not yet known whether the rebels had suffered any casualties.

But they failed to snatch weapons as the policemen who survived the initial attack fought back.

It was not immediately clear which rebel group was responsible for the attack.

However, intelligence officials said they suspected the involvement of the separatist United National Liberation Front, which has been fighting for Manipur's independence since 1964.