Fien Riske-Nielson, UN Humanitarian Coordinator in East Timor, said on Monday that the world body would issue a "flash appeal" for aid and funds later on Monday at its New York headquarters for $18 million to cover three months of operations.
He said he expected "a positive response to the appeal."
Riske-Nielson also said the number of internally displaced persons who fled their homes after violence last month was now estimated at 100,000, scattered in various camps around the capital Dili and nearby districts.
"We are planning to do an assessment again in the coming days because there are indications that the number could be increasing outside Dili," he said as he toured an overcrowded refugee camp run by Catholic nuns.
The humanitarian situation, he said, remained "precarious" because even if the situation was under control for the time being, the overcrowding in camps could lead to outbreak of disease.
"The situation in the camps is very difficult," he said. "People aren't going home because they are afraid."
Jose Ramos-Horta, the foreign minister, also speaking at a refugee camp, said he hoped refugees would begin returning to their homes "if they feel the situation is really stabilised."
The UN announcement came with rebel soldiers and the government still in deadlock over calls for Mari Alkatiri, the prime minister, who has been widely blamed for the violence, to resign from office.
President Xanana Gusmao has been leading efforts to try and break a deadlock with rebel soldiers who are calling for Alkatiri to quit and prevent further escalation of tension, the foreign minister said.
Alkatiri in March sacked 600 soldiers from the country's west after they deserted their barracks complaining of discrimination.
"The situation in the camps is very difficult. People aren't going home because they are afraid"
Fien Riske-Nielson, UN Humanitarian Coordinator in East Timor
The sacking triggered fighting between rival soldiers and soldiers and police that quickly disintegrated into street violence involving rival gangs.
Political opponents of the prime minister are to launch a meeting this week to come up with a formula for modifying the constitution to give Gusmao greater powers.
It would "take a few more days or even weeks before there is absolute clarity of what direction this country is taking" ahead of elections next year, Ramos-Horta said.
"President Xanana has met with some of the ex-military (soldiers) and are fully in touch with them," he said.
The UN's refugee agency said some 150,000 tonnes of relief supplies were to arrive later on Monday from Australia.
The cargo was airlifted from the agency's warehouse in Jordan last week and includes lightweight family tents, plastic sheets, jerry cans and kitchen sets.