At least three BSF jawans and one civilian were killed on Wednesday morning in a landmine blast, triggered by Maoists in Odisha's Malkangiri district.
Six others jawans were severely injured in the attack and were rushed to the Malkangiri district hospital.
Maoists ambushed BSF personnel while the latter were conducting a combing operation in the Chitrkonda area of the district, about 700 km southwest of Bhubaneswar.
Police and BSF have started an extensive search in the area for the Maoists.
Malkangiri is the rebel's main stronghold in the state. This is the first major attack on security personnel in Odisha in the last three years.
A Central Reserve Police Force (CRPF) jawan was killed and two were injured in an encounter with the outlawed CPI (Maoist) at Timapur in Bijapur district of Chhattisgarh last week.