Another five soldiers and policemen were wounded in the explosion at Ghorahi town, 400km west of Kathmandu, the Himalayan kingdom's capital, on Thursday.
Hem Raj Sharma, a local journalist, said one civilian was also wounded in the blast.
The latest attack came days before a Maoist initiative to enforce an indefinite blockade of Kathmandu, a hill-ringed city, and other large towns to bring their rebellion to the cities.
The Maoists' blockade, threatened from 14 March, is seen as an attempt to increase pressure on King Gyanendra, who seized absolute power last year and vowed to crush the rebels who want to topple the monarchy.
More than 13,000 people have died in the conflict since 1996, including 200 who have been killed this year.