None was injured in the encounter, the police said.
The insurgents hurled the grenade at an SSB patrol at Phoisana Bazar area under Bijni sub-division and traded fire with them before fleeing into the nearby thick jungles, they said.
CRPF personnel from nearby Panbari camp rushed to the site and jointly launched an operation with Sashastra Seema Bal to trace the fleeing extremists.
Meanwhile, Jharkhand's former Chief Minister Babulal Marandi arrived here today to meet the people affected in the NDFB-S carnage in the district and neighbouring Kokrajhar district.
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Demanding security to the people and that the army stop NDFB-S activities, he wanted the violence-affected people sheltered in relief camps be provided with food, clothes, medicines and other essential items.
State Welfare minister Akon Boroa visited the district today and assured the inmates in the relief camps of food and other essential goods.
At least 81 people were killed in attacks by NDFB-S and retaliatory violence in Assam's Kokrajhar, Sonitpur and Chirang districts since December 23.