The attack took place in the Mahottari district on Sunday, about 400km east of Kathmandu, police said.
At least one of the victims was a police inspector, but it is not known whether he was the specific target.
“The truck blew up and was thrown about 10 metres in the air before it came crashing to the ground,” Mahottari district administrator Bimal Dhakal told journalist by telephone.
In the Nepalese capital, Kathmandu, 13 Maoist rebels were also killed in an escalation of violence, officials said on Sunday.
At least five Maoist rebels were killed in a gunbattle with troops in Dailekh district – about 550km west of the capital.
In separate operations, soldiers gunned down eight guerrillas and the Maoists killed a farmer in the past 48 hours, the defence ministry confirmed.
More than 8100 people have been killed in the seven-year revolt that has wrecked Nepal’s aid and tourism-dependent economy, triggered political turmoil and damaged infrastructure.
Battling Nepal’s constitutional monarchy, rebels seeking to establish communist rule walked out of peace talks and broke a truce last August.
The move ended seven months of calm in the impoverished Himalayan kingdom.
Four million affected
Nearly four million Nepalese have been directly affected by the insurgency. The country’s populaton is 24 million.
The Kathmandu-based human rights’ group Informal Sector Service Centre in a report, claims people have suffered either physically or economically in the violence that has swept the impoverished country since 1996.
Most people have known someone killed or wounded in the conflict.
But the group said 18,000 Nepalese were currently displaced and that since 1996, about 400 people have “disappeared,” including a number of non-Maoist local leaders abducted by the rebels.