The UN says armed conflict could resume in Nepal if the government does not join the unilateral ceasefire declared by the Maoist rebels.
It is not clear if the rebels will renew their ceasefire
The UN's human rights chief, Louise Arbour, voiced her concern on Thursday, a day before the three-month-old rebel ceasefire is due to expire.
Nepal's King Gyanendra is scheduled to return from a foreign tour on Friday.
About 12,000 people have died in Nepal's 10-year insurgency which is aimed at replacing the monarchy.
National and international pressure has been mounting on the Maoists to extend the truce and on the government to reciprocate, says the BBC's Sushil Sharma in the capital Kathmandu.
In a statement, Ms Arbour said: "I am seriously concerned about the very real possibility that full-scale armed conflict could resume."
Our correspondent says that fears of renewed violence have been heightened by the recent robbing by rebels of a group of journalists in the eastern hill district of Taplejung, followed by the killing of two senior rebel leaders by government troops in the western district of Rolpa.
The government believes rebels are rearming and regrouping
Following these incidents the top rebel leader, Prachanda, accused the government of provoking the rebels into breaking the truce.
The government has refused to reciprocate the ceasefire saying the rebels are using this time to rearm and regroup.
Our correspondent says there has been persistent international pressure - from countries like India, the UK, the US along with the UN Secretary General Kofi Annan - to end the violence.
If the Maoists extend their truce, there will be added pressure on the government to clear the way for a negotiated settlement to the decade-long conflict, analysts say.
The monarchy in Nepal has lost much of its popularity after the king seized direct powers in February this year.
Recently, the rebels and a coalition of seven opposition parties agreed on a programme designed to end direct rule by King Gyanendra and restore democracy.