The slain militant was identified as KLO Kokrajhar 'district commander' Samar Roy alias Rambo (27) who carried a cash reward of rupees five lakh on his head, IGP L R Bishnoi said.
The encounter occurred in a forest area of the district at around 3 pm when three militants exchanged fire with the police in which the militant was killed, the IGP said.
A pistol, eight rounds of bullets, two hand grenades and two mobile hand sets were recovered, Bishnoi said.
Roy was involved in a firing incident upon a labourer at Fakiragram of Kokrajhar district yesterday, the IGP added.