The forest guards deployed in the Burapahar southern range of the 430 sq km KNP heard gunshots around 1.30 am and they immediately replied with bullets, Park authorities said.
An encounter ensued but the poachers decamped with the horn of the slain rhino, they said.
The forest guards informed the Park authorities through wireless and additional patrolling parties were rushed from the headquarters.
On launching a search operation, the carcass of the slain rhino with four bullet wounds and its horn cut and removed was recovered around 4.30 am, the sources said.
The horns of the Indian rhinos are preferred over African rhinoceros horn as they fetch five to ten times higher price selling for up to Rs 1.5 million per kg in the clandestine international market, forest department sources said.
Rhino horns also find a market in the Middle East where they are used as decorative handles of ornamental daggers, they added.
The Great Indian Rhinoceros, found in Kaziranga and the Pobitora Wildlife Sanctuary near Guwahati, is listed under Schedule I of the Wildlife (Protection) Act 1972 and Appendix I of Convention on the International Trade in Endangered Species of Wild Fauna and Flora.
Both the legislations prohibit domestic and international trade in the species.
Meanwhile, the Assam government has recently entrusted the CBI to investigate the sudden increased incidence of rhino poaching in KNP, Pobitaora which has the highest density of rhino population in the world and Orang National Park where rhinos were translocated from KNP.
Altogether 1,855 of the world’s estimated 2,700 rhinos are inhabitants of the world famed Kaziranga National Park.