Huge cache of arms and ammunitions was also seized in the operation, police said.
Following an information about presence three ULFA militants in Thanti Hills, a joint team of the army and police launched the operation which led to the encounter.
The militants first opened fire at the security personnel and an exchange followed in which a militant was killed on the spot while another was injured, though he managed to escape along with the third, police said.
The security forces later recovered two Improvised Explosive Devices (IED), a 7.65 mm pistol, three live ammunitions, five empty cartridges, a wireless set, some gelatine and explosives along with several extortion notes and documents.
