The encounter began at 12.30 pm at Ratani Pathar under Pengeri police station where combing operations were on since Sunday.
The dead militant was identified as Mohipal Moran. There were unconfirmed reports of injuries to five army personnel, sources said.
A pistol and a grenade were also recovered from the slain militant and the entire area leading to Pengeri was cordoned off.
An encounter also took place at Pengeri on Sunday but there was no casualty. However, one ULFA militant was killed during in a shootout in neighbouring Digboi area.
The army has launched massive operations in the Upper Assam districts of Tinsukia and Dibrugarh since Sunday after the Centre called off the ceasefire following the killing of Hollonghabi tea estate's garden manager on Saturday near Digboi.
The Centre has ordered the Army to resume operations, suspended since August 13, against the militant outfit.
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