Maoists ambushed a joint patrol at Sipkhola area in Mathura Danda village of Arghakachi district, 450 km west of Kathmandu, state-run Nepal Television reported.
The firefight started at 8 am and continued for several hours.
At least six Maoists rebels died when the security forces retaliated, and reinforcements have been rushed to the area for a cordon and search operation.