Maoist guerrillas and government forces in Nepal fought a pitched battle that killed scores overnight, as US President George W. Bush offered aid to the Himalayan kingdom.
The Maoists attacked a security post in their remote stronghold of Gam in the west, where troops killed up to 400 guerrillas last week in the biggest single offensive of the six-year rebellion.
"The raid came about midnight and we seem to have lost heavily," one army officer, who asked not to be named, told Reuters.
Communications have been cut and bad weather has prevented rescue helicopters from landing, senior army officers told Reuters, adding reinforcements were on the way.
But they said up to 100 rebels, police and soldiers were killed in the fierce firefight, about 450 kilometres west of the capital, Kathmandu.
Interior Minister Khum Bahadur Khadka told Reuters in Kathmandu the government was awaiting detailed casualty figures.
"There were more than 140 soldiers and police officers at Gam and we have lost contact with them," he said, adding he could not say how many had died.
At least some police and soldiers were known to have survived the clash before communications were lost, the army said.
More than 4,000 people have been killed in the rebellion aimed at toppling the impoverished Hindu kingdom's constitutional monarchy and installing a communist republic.
US military experts visited Nepal recently to assess the impoverished South Asian nation's military needs and to gauge what support Washington could provide, officials said. The Bush administration is seeking $20 million from Congress to help Kathmandu fight the guerrillas.
The violence has wrecked the aid-dependent economy and driven away tourists, who are a key source of income to the picturesque nation that is home to Mount Everest and draws thousands of backpackers and mountain climbers each year.