India has imposed a curfew in parts of the northeastern state of Assam after suspected tribal guerrillas killed at least 50 people in a series of attacks in retaliation for an offensive against them.

Assam has a history of sectarian bloodshed and groups fighting for greater autonomy or secession from India.

"As of now 56 people are dead and 80 others are injured. At least 20 of them are in critical condition in hospitals," SN Singh, Assam Police inspector-general, told AFP news agency.

Tuesday's attacks in four places by fighters of the outlawed National Democratic Front of Bodoland within the space of an hour were the deadliest in months.

Indian security forces launched a campaign last month against the rebels in their remote hideouts, prompting a threat from them to target settlers.

Reports said the attackers opened indiscriminate fire at Saralpara village in Kokrajhar district and in Santipur village of Sonitpur district.

The attacks came after a joint team of Assam Police and the Indian army killed two Bodo fighters in the neighbouring Chiran district on Sunday.

Villagers told police the rebels came on foot, armed with assault rifles and wearing military uniforms.

"They didn't even spare women and children," said a police officer, adding there were at least 10 women among the dead in Tuesday's violence.

At least 13 children were killed.

"There is a curfew imposed by the police in a number of districts of Assam," Al Jazeera's Karishma Vyas, reporting from New Delhi, said.

"Police believe that this attack was planned. The gunmen reportedly fired indiscriminately on these villages that are largely populated by people know as the Adivasis."

Culture under threat

Some members of the Bodo community say their identity, culture and language are under threat of being overrun and the only way they can preserve it is through self-governance.

The rebels say they are fighting for a separate homeland for the indigenous Bodo people. They complain that the tea-growing state has been flooded with outsiders.

Police Inspector-General SN Singh said in Guwahati, Assam's capital city, that additional forces were being sent in to chase down the attackers. He said villagers at risk would be evacuated to safer places.

Lalit Gogoi, deputy commissioner of the worst-affected Sonitpur district, said the army had been put on standby and a dusk to dawn curfew imposed.

The single hospital in Sonitpur was crowded with scores of people with bullet wounds.

Assam is one of seven states in India's northeast, a region bounded by China, Myanmar, Bhutan and Bangladesh. For long, residents have accused the central government of plundering its natural resources and ignoring development.

Narendra Modi, Indian prime minister, condemned the attacks in a Twitter post, saying that the killings of innocent people were an "act of cowardice".

Modi has pledged to accelerate the development of roads and railways in the area.

Source: Al Jazeera and agencies