A hardcore NDFB(S) militant was killed at an encounter with rival faction in Udalguri district on Wednesday. The encounter took place at Rowta where an ultra died on the spot. Identified as Nirjon Basumatary alias Ganda alias B Nangtha, the NDFB(S) man was killed when two groups of the militant outfits clashed with each other. Three civilians were also injured in the crossfire. They have been undergoing treatment at the Gauhati Medical College Hospital. They were identified as Lalit Swargiary (30), Pratap Boro (24) and Ratneswar Boro (22).