According to a police official, at least 600 rebels attacked a police post late on Friday ending a temporary truce to mark the Hindu festival of Dasain. "The bodies of 50 rebels have been recovered but many others were carried away by the insurgents themselves,” police said.
It was one of the biggest clashes since the guerrillas withdrew from a seven-month ceasefire in Argust.
As armed police reinforcements reached the site, the rebels fled into the nearby jungles.
Khas-Kusum is close to the southwestern town of Nepalgunj, a rebel stronghold.
The Maoists had declared a temporary eight-day truce, which ended on Thursday, to mark the Hindu kingdom’s biggest festival Dasain, celebrating the victory of the goddess Durga over the forces of evil.
The rebels, who want to overthrow the monarchy and set up a communist regime, pulled out of the seven-month ceasefire and peace talks on August 27 when the government refused to call a special assembly to redraft the constitution.
More than 8100 people have died in the Maoist insurgency since 1996, government figures show.