On a tip off about the entry of a group of poachers into the Park, forest guards launched an operation in Burapahar Range, Divisional Forest Officer SK Sil Sarma said.
On facing the forest guards, the poachers fired at them and in an encounter one poacher was killed around 3 am near Borghat forest camp, Sarma said.
On being chased, the two fleeing poachers engaged in a fierce gun battle with the guards for about 25 minutes before they were killed, he said.
Two 303 rifles fitted with a silencer and one round of ammunition as well as a 9-mm pistol with nine rounds of ammunition were recovered from the slain poachers, the DFO said.
The poachers were suspected to have come from adjacent Karbi Anglong hills side, he said.
Patrolling against poaching in the World Heritage Site had been intensified after one rhino was killed on 1 January this year and two on 4 January, a forest department official said.