Acting on a tip-off, troops of Red Horn Division and the police launched a joint operation in a dense forest near Kokrajhar and killed the four militants during an encounter this morning, a defence spokesman said.
The forces recovered an AK-56, three pistols and hand grenades from the slain terrorists.
The slain militants include a self-styled second in command of the outfit, Muhar Basumatary.
Assam Police Inspector General (BTAD) L R Bishnoi said the other ultras were identified as Domsatorong, Loharun and Maisaran.
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According to Army Intelligence, NDFB(S) was trying to carry out a major strike on forces and also target minority communities in retaliation to the apprehension of their five hardcore terrorists in Chirang district on August 20.
Security forces had apprehended five terrorists of a newly-formed Nepalese terrorist organisation, United Gorkha Peoples Organization, in Parbatjhora area of Kokrajhar district on September 13.
The Army spokesman claimed the NDFB(S) had outsourced the outfit to carry out operations in Kokrajhar district because of proactive operations and pressure exerted by the security forces.