On a tip off that cadres of the outfit assembled near Longkai village, Khonsa battalion of the Assam Rifles launched an operation.
In the ensuing encounter, one of the cadres were killed and his body was handed over to the police.
He was identified as a "captain" of the Nationalist Socialist Council of Nagaland (Issac-Muivah).
Two AK-56 rifles and a large quantity of ammunition were recovered from the site of the encounter, police said.
The security forces have launched a search operation in search of other militants.
In another operation, he Khonsa battalion captured a cadre of the NSCN (Khaplang) from Chasa Village in Tirap district of the state Thursday morning, police said.
Three pistols, some ammunition, opium and cash were recovered from his possession.
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