Police were patrolling Dhankhola village, about 400km southwest of the capital, Kathmandu, when their vehicle was struck on Saturday by a landmine, said police officials reached by telephone in the nearby Dang district.
Rebels then opened fire on the police officers. At least four civilians on a bus that was travelling right behind the police truck were also killed.
Police said they didn't have further details yet.
Fighting between the rebels, who say they are inspired by Chinese revolutionary leader Mao Zedong, and government forces has escalated since the guerrillas withdrew from a seven-month ceasefire last August.
The rebels have been fighting since February 1996 to replace Nepal's monarchy with a communist state. The insurgency has claimed more than 9500 lives.