Santhal tribesmen killed two couples and a teenage girl in two separate incidents in the Kokrajhar district.

The villagers said the couple were practising witchcraft and harming fellow tribes people, the police said.

Analysts say villagers are often accused of witchcraft to settle personal and land disputes.

Originally from central India, the Santhal tribes people were brought to Assam during British rule to work in the tea gardens.

Real reasons

The BBC's Subir Bhaumik in Delhi says the Santhals are generally found in the states of Bihar, Jharkhand, West Bengal and Assam and it is quite common among them to attack people they suspect of practising witchcraft.

One study shows a number of cases in West Bengal and Assam where certain families have been attacked by rivals within the community for practising witchcraft and their land taken away after the killings.

"The Santhals fear witches and believe great harm can be done by them," says the study by senior police officer Asit Baran Choudhury.

"So anyone accused of practising witchcraft can come in for severe punishment and this is often manipulated for settling personal scores," he says.

According to police records, some 200 people have been killed in Assam in the past five years for allegedly practising witchcraft.