BANDA ACEH, Indonesia - Masked workers with mosquito-killing spray guns began moving through refugee camps today in tsunami-battered Aceh province, trying to prevent an outbreak of malaria. Indonesia, meanwhile, said it is pursuing a permanent truce with rebels in the area, the worst-hit by the disaster.

While the threat of cholera and dysentery is diminishing because clean water is reaching tsunami survivors in Indonesia, the danger of malaria and dengue fever epidemics is increasing, according to the leader of anti-malaria efforts in the region.

"Short-term, we're trying to prevent an epidemic," said Richard Allan, director of the Mentor Initiative, a public health group that fights malaria epidemics. "And it may already be too late."

The death toll from the Dec. 26 earthquake-triggered tsunamis in 11 countries has topped 157,000. Two-thirds of the deaths occurred here in Indonesia's Aceh province.

Allan warns that 100,000 more people could die of malaria around Aceh if quick action isn't taken. A successful spraying effort would drastically minimize that risk.

The pools of salt water created by the tsunamis have been diluted by seasonal rains into a brackish water, creating the perfect breeding ground for mosquitos.

A fumigation operation started today with a small team of sprayers planning to cover up to eight refugee camps around the provincial capital of Banda Aceh. The main spraying effort won't begin for at least two days, because most of the insecticide has been held up by bureaucratic delays in Jakarta, he said.

Poor health and tsunami-related stress has weakened the immune systems of the displaced people, leaving them abnormally susceptible to mosquito-borne illness, said Allan.

Indonesian Vice President Jusuf Kalla, visiting Banda Aceh, said his government is working on a full truce with rebels who have been fighting off and on for an independent homeland in northern Sumatra for 30 years.

Exiled rebel leaders in Sweden called a unilateral cease-fire the day of the tsunamis and have in recent days called for peace talks.

"We are not going for a cease-fire. We are making it permanently and we are working for that," said Kalla, who did not say whether negotiations already were under way.

Despite the talk about a cease-fire, the Indonesian government has insisted that foreign aid workers in Aceh be accompanied by army escorts - a move that relief groups say will hinder their work.

In Sri Lanka, more than 25,000 people displaced by the Dec. 26 tsunamis left relief camps in the past 24 hours to return to rebuild their villages, the United Nations' refugee agency said today.

"Some want to return because their houses were partially damaged and some want to return to where their house was to be sure that they are recognized" as the owner, said Neil Wright, an official with the United Nations' High Commissioner for Refugees.

U.S. helicopters flew into eastern Sri Lanka today, ferrying some 30 tons of relief materials, including fresh fruits and vegetables.

"We're concentrating on the eastern areas where many people were stranded after their roads and bridges were destroyed," said Sri Lankan Wing Commander Senaka Dharmawardene, who is in charge of coordinating aid in the eastern region. "(The Americans) have been very helpful."

The tsunamis killed 31,000 people in Sri Lanka and rendered 800,000 people homeless. Wright said more than 425,000 still remain in refugee camps.