KOLKATA: Four people were killed when the crude bombs they were making exploded in India's West Bengal, where bitterly fought state elections are underway, a senior police official said Monday.

Another six people were injured in the explosion on Sunday night in a village about 360 kilometres north of Kolkata, close to the eastern state's border with Bangladesh.

"Four people were killed and six others injured when the bombs they were making exploded in a house at Baisnabnagar village," Anuj Sharma, West Bengal additional director general of police, told AFP. "The explosion rocked the area and damaged the house."

Police were investigating the reasons for the bombmaking but no arrests have been made so far, Sharma said.

West Bengal is set to hold the final phase of staggered state elections on Thursday. Homemade bombs are sometimes planted by workers from rival political parties to intimidate voters ahead of polling day.

India has sent extra security forces to West Bengal for the election.

Chief minister Mamata Banerjee is battling left-wing rivals and the Hindu nationalist Bharatiya Janata Party, which rules at national level. Banerjee's Trinamool Congress party has also been hit by bitter infighting among its rival factions.

Two people were killed in March when their homemade bombs accidently exploded, also in West Bengal, while police in February recovered 80 crude bombs from the house of a local leader of the Trinamool Congress party, according to local media.