A militant identified as Nirjon Basumatary alias Ganda alias B Nangtha was killed when two groups of the militant outfit fired at each other at Rowta in Udalguri district last night, a police officer said.
Three civilians were also injured in the crossfire, he said.
The injured were admitted to the Gauhati Medical College Hospital and they were identified as Lalit Swargiary (30), Pratap Boro (24) and Ratneswar Boro (22).
Basumatary was admitted to Tezpur Medical College Hospital, police said.
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The encounter took place at Laigangpara in Batachipur area of Sonitpur district along the Assam-Arunachal Pradesh border.
A 7.65-mm pistol, one magazine, two grenades, two 9-mm cartridges and one detonator were recovered from the militants, police said.
Besides Basumatary, three ultras were also nabbed.
Security forces have started operations near the Assam-Arunachal Pradesh border areas after the incident.