Indian army patrols Assam state

The army has mounted an offensive against suspected separatists

Indian army soldiers are patrolling districts of Assam state after at least 55 people were killed in two days of attacks by suspected separatists.

Police say the United Liberation Front of Assam (ULFA) is responsible, but the rebel group has yet to respond.

A permanent curfew has been imposed in the Tinsukia district following a spate of attacks on Hindi-speaking settlers.

The government has sent a high-level team to assess the worst violence seen in the state for at least a decade.

'Indiscriminate killing'

Officials said security forces were mounting an "all out" offensive against the separatists, aimed at flushing them out of their camps in Assam's jungles.

"Massive combing operations have started and additional troops are being rushed to the affected areas," Assam's chief minister Tarun Gogoi told Reuters news agency.

Police said the rebel group attacked six colonies of Hindi-speaking labourers in the northern districts of Dibrugarh and Tinsukia within three hours.

Witnesses described groups of 10 to 15 masked and heavily armed men firing indiscriminately, sparing none - not even women and children.

By late on Saturday, 48 Hindi-speaking settlers had been killed, making it the worst carnage of migrants in Assam for at least a decade.

The authorities have imposed a round-the-clock curfew in the Tinsukia district near the border with Burma, after thousands of angry residents protested against the failure of the security forces.

In the latest incident early on Sunday morning, a railway bridge was blown up just after a train had passed over it, says the BBC's Subir Bhaumik in Assam.

There were no casualties, although railway officials said one carriage was seriously damaged.

In an earlier incident nearby, five policemen and two government officials were killed when a landmine exploded in the district of Karbi Anglong.

Indian Prime Minister Manmohan Singh condemned the killings as an "act of cowardice and inhumanity".

'On the defensive'

Our correspondent says ever since talks between the ULFA and the government collapsed in September, the rebels have targeted Hindi-speaking migrants, mostly from the northern state of Bihar, with bomb and grenade attacks.

Officials say the ULFA has intensified such attacks to pressure the government into calling off its military operation against the rebel group, or at least put it on the defensive by forcing it to guard target villages.

Security officials say attacks could intensify ahead of India's Republic Day celebrations on 26 January. The ULFA has also called for a boycott of next month's Indian National Games.

The rebels are seeking a separate homeland for the Assamese people and demanding the departure of the non-indigenous population, particularly Hindi speakers.

They have been fighting Delhi's rule in the tea and oil-rich state for the past 27 years. At least 10,000 people have died in the violence.