Maoist rebels in Nepal have killed 29 soldiers and police east of the capital, Kathmandu, authorities say.
Security has been stepped up since the ceasefire collapsed
At least 10 rebels are also reported to have died in the overnight clash in remote Bhojpur district.
Officials say hundreds of Maoists attacked security forces guarding a telephone tower, severing communication links with the rest of the country.
The rebel attack is the most serious since peace talks aimed at ending eight years of civil war failed last August.
Almost 9,000 people have died in violence in the kingdom since the Maoists began their rebellion to replace the monarchy with a communist republic.
Troops and rebels exchanged fire for several hours in Bhojpur, a Maoist stronghold 500km (315 miles) east of Kathmandu, reports say.
The office of a local administrator and a bank were also attacked. The telephone tower was destroyed.
The rebels reportedly fled after an army helicopter with night vision arrived to back up soldiers and police.
"Soldiers searching the area after the fighting have
recovered 10 bodies of Maoists," an army official told Reuters news agency.
"Communication links with the area have been cut off."
Officials say at least seven other soldiers were injured during the assault. A further 10 soldiers have been reported missing.
The casualties sustained by the security forces are the biggest since hostilities resumed last August, the BBC's Sushil Sharma in Kathmandu says.
He says the rebels had appeared to have been pushed onto the defensive in recent months.
The Maoists have been blamed for a string of recent minor bomb attacks in Kathmandu, and have called a series of crippling strikes.