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

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

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

"The militants first came and asked for water. Suddenly they opened fire with their AK-47 rifles,” a witness, who fled into jungle, later told reporters.

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

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

"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.

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 gunshot 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 federal government of plundering its natural resources and ignoring development.

Prime Minister Narendra Modi has vowed to accelerate the development of roads and railways in the area.