The violence came on the seventh day of a blockade around the capital Kathmandu by the Maoists, who control much of the countryside and are trying to overthrow the monarchy.
The Maoists set off a landmine at Chhahare, about 80km northeast of Kathmandu, killing four soldiers and injuring two who were flown to the Nepalese capital for treatment, an army official said on Tuesday.
The official also mentioned that rebels were seen carrying about 15 bodies. However, he could not say if they were injured or killed.
Rebels had so far relied on the fear of intimidation rather than an armed presence on the highways to keep Kathmandu cut off from the rest of the country.
About 10,000 people have died in Nepal since the Maoists launched their "people's war" in 1996.