Nepal's armed forces have launched a major offensive against Maoist rebels in the south-western mountainous region, authorities in Nepal have said.
The UN says civilians are being endangered by the fighting
Officials said army helicopters were mounting bombing raids on the Palpa and Nawalparasi districts.
Violence has escalated in Nepal after the rebels called off a four-month unilateral ceasefire last month.
The United Nations says civilians are being placed in grave danger with the resumption of conflict.
In a report by its chief human rights official, Louise Arbour, the UN says over the past month clashes in highly populated areas had endangered civilian lives.
"Children in particular have been placed at risk, including as combatants within the... Maoists, by indiscriminate action by the security forces and by the placement or abandonment of explosive devices," Ms Arbour said in her report.
The report will be presented to the next session of the UN Human Rights Commission next month.
The latest offensive follows a rebel attack on an army convoy in Palpa district on Wednesday.
The government believes rebels behind recent attacks on government and civilian targets are hiding there.
Three soldiers were killed in the clashes.
An army official told the BBC that the rebels had suffered heavy casualties. The rebels have not commented yet.
The latest attack followed controversial local elections last week in which only about 20% of voters cast ballots amid an opposition boycott and rebel threats.
The rebels have launched major attacks in Palpa and neighbouring Nawalparasi district over the past two weeks, leaving scores of soldiers, police and rebels dead.
There has been a spike in violence across Nepal after the rebels called off a ceasefire last month.
The government had refused to tell soldiers to stop offensives, saying it did not trust the rebels.
The rebels ordered a nationwide shutdown to disrupt the municipal elections, which King Gyanendra says will precede parliamentary elections next year.
The opposition and the rebels said the local elections were a sham to entrench the power of the king, who seized direct power in 2005.
Some 13,000 people have died in the 10-year-old Maoist insurgency.
Top Maoist leader Prachanda told the BBC, in an interview broadcast on Monday, that the only future he saw for the king was exile or trial, possibly leading to execution.