The United States has ordered all its non-essential diplomatic staff and their families to leave Nepal.
The US is worried by increasing tension in Nepal
The move comes on the eve of what protesters in the capital, Kathmandu, vow will be their biggest demonstration yet against King Gyanendra.
A new curfew has been imposed in the capital to stop further anti-king protests in which 14 people have died.
Meanwhile, a number of people have been killed in fighting between security forces and Maoists near Kathmandu.
US officials in Kathmandu say the state department order for non-essential staff to leave Nepal reduces the number of staff at the US mission in Nepal by about a half.
They are reported to be concerned about dwindling food and medical supplies and the sometimes "violent measures" used by the authorities to break up the protests.
Earlier this month the US said that non-emergency staff and their families could leave at their own discretion.
Embassy spokesman Robert Hugins said those ordered to go on Monday would leave the country as soon as possible, the Associated Press news agency reports.
The state department is also advising all other US nationals in Nepal to leave the country.
Overnight, hundreds of Maoists stormed the town of Chautara, about 125km (75 miles) east of Kathmandu, and fought security forces for six hours.
Correspondents say that it was the first major rebel attack on government targets since the seven-party opposition alliance launched its anti-king protests.
The rebels targeted government buildings including the army base, the police post, the district prison and district headquarters.
The BBC's Charles Haviland - who went to the scene - says there was a lot of blood shed and the village hospital was a battleground.
Its maternity ward is covered in bloodstains, he says, and buildings and compounds are strewn with improvised bombs, many of which have yet to go off.
The army say the rebels attacked the hospital because they wanted to throw bombs at the heavily guarded telecommunications tower next door.
Our correspondent says that the attacks are an attempt to show the Maoists are strong, but in Chautara they have been repulsed, albeit with some difficulty.
The remote and mountainous area is said to be a stronghold for the rebels who control large swathes of the Nepalese countryside.
Meanwhile in Kathmandu the seven-party opposition alliance opposed to the king says it is hoping that half a million people will attend a rally on Tuesday.
A fifth consecutive curfew came into force in Kathmandu at 1100 (0515 GMT) on Monday as the country entered its 19th day of demonstrations against King Gyanendra's direct rule.
Correspondents says that protesters still took to the streets, setting up roadblocks and burning tyres, but the army and police - who are everywhere present - kept their distance.
The king seized power in February, 2005, saying the elected government was failing to deal with the Maoist rebels.
The opposition parties say the king's recent invitation for them to form a government does not go nearly far enough, as the monarch would still be free to fire prime ministers at will.
They want an elected assembly to write a new constitution and also a process to draw the Maoist rebels into peaceful politics.
"We rejected the king's offer because we believe he wants to form a government that he can control. This is just a ploy to try and extinguish our struggle."
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On Sunday, police again fired rubber bullets and tear gas to disperse tens of thousands of demonstrators.
More than 13,000 people have died in the 10-year Maoist insurgency aimed at replacing the monarchy with a communist republic.