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A senior Islamic leader warned foreign relief workers Friday of a serious backlash from Muslims if they bring Christian proselytizing to tsunami-struck Sumatra (search) along with humanitarian help.

Masked health workers, meanwhile, fanned out spraying insecticide to kill mosquitos and prevent malaria (search) from breaking out in Aceh province's refugee camps, where poor sanitation and contaminated water pose a health risk to tens of thousands of survivors.

U.N. Secretary-General Kofi Annan (search) said he would name a special envoy next week to coordinate relief and reconstruction in the 11 countries hit by last month's earthquake and tsunami that killed more than 157,000 people, two-thirds of them in Indonesia.

Annan, speaking to reporters at a conference in the Indian Ocean nation of Mauritius, did not explain how the envoy's role would differ from that of the U.N. emergency relief coordinator, Jan Egeland, who has been responsible for coordinating tsunami aid.

On Saturday, U.S. Deputy Secretary of Defense Paul Wolfowitz (search) met with a Thai Defense Minister Gen. Samphan Boonyanant to discuss relief efforts, including the $350 million Washington has pledged. Wolfowitz also planned to visit hard-hit areas of Indonesia, Thailand and Sri Lanka. Details of the private session weren't immediately disclosed.

At Friday prayers in the main mosque of Banda Aceh, the provincial capital, a Muslim leader warned against any attempt by Christian aid workers to evangelize among tsunami survivors. Indonesia is the world's most populous Muslim nation, and Aceh is particularly conservative.

"All non-governmental organizations, either domestic or international, with hidden agendas coming here with humanitarian purposes but instead proselytizing, this is what we do not like," said Dien Syamsuddin, secretary-general of the Indonesian Council of Ulemas, or religious scholars.

He also condemned reports the U.S.-based welfare group WorldHelp had planned to adopt 300 Acehnese children orphaned by the disaster and raise them in a Christian children's home.

The group told The Associated Press on Thursday it had dropped the idea.

"This is a reminder. Do not do this in this kind of situation," Syamsuddin said. "The Muslim community will not remain quiet. This a clear statement, and it is serious."

Later Friday, teams with insecticide sprayers began working in refugee camps around Banda Aceh, where the tsunami and heavy rains have left pools of stagnant water that are perfect breeding grounds for mosquitos.

"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."

Allan warned that an additional 100,000 people could die of malaria in the Aceh region if quick action wasn't taken to reduce the numbers of mosquitos.

Other major health risks in Aceh included dirty drinking water — often from unsanitary latrines — that could give people cholera, typhoid, dysentery and other waterborne diseases.

While the chances of an outbreak are diminishing as more clean water reaches survivors, the danger is not over and epidemics could erupt at any time, health experts say.

In Sri Lanka, another hard-hit nation, there were signs of resilience as more than 25,000 people left relief camps over the preceding 24 hours to return to their villages and begin rebuilding, U.N. officials said Friday. They said just over half the 800,000 Sri Lankans left homeless by the tsunami remained in camps on the island, where the waves killed 31,000.

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

In Aceh, Australian troops ferried heavy earth-moving and electrical equipment, water-purification materials and other supplies from a navy frigate.

"It's a significant step," said Brig. David Chalmers, commander of the nearly 1,000-strong Australian contingent sent to Indonesia.

During a visit to Banda Aceh, Indonesian Vice President Jusuf Kalla told reporters the government was pursuing a permanent truce with Acehnese rebels who have been fighting for an independent homeland in northern Sumatra for decades.

Malik Mahmud, a leader of the Free Aceh Movement exiled in Sweden, welcomed the move. But when asked if they would drop their independence bid, he replied: "The struggle is deep in our hearts."

Despite the talk about a cease-fire, Indonesia's government is insisting foreign aid workers in Aceh be accompanied by army escorts — a move relief groups say will hinder their work. Indonesia is sensitive about foreign involvement in the area, and reiterated Friday that it wanted foreign troops out by late March.

Wolfowitz told reporters during his flight to Asia that he hopes the U.S. military's role in the relief mission will be finished well before the end of March.

"For any country it is sensitive to have foreign troops on your territory. It would be sensitive in the United States and I can tell you that it is extremely sensitive in Indonesia," he said. "What's remarkable is that it has caused no problems to date."

About $92 million of the promised $350 million in U.S. aid pledge for the tsunami relief assistance has been spent so far, given to U.N. organizations and private relief groups, Tom Fry of the U.S. Agency for International Development said Friday in U-tapao.

The rest of the $350 million will be allocated as relief organizations submit proposals for specific projects, he said.

In Tokyo, a Japanese official said Japan and the United States, which have the most advanced tsunami alert systems, would provide tsunami warnings to countries around the Indian Ocean as a provisional measure until the region can set up its own system. Experts have said casualties could have been substantially reduced if there had been warning of the approaching waves.