With health officials warning that the death toll of more than 140,000 from last month's tsunami could jump sharply without a continual supply of aid, world leaders struggled Thursday to figure out the best way to help victims — and to prevent such a catastrophe from happening again.

Donors concluded an emergency one-day summit as relief workers scrambled to move aid to areas of Sumatra (search), the Indonesian island hit hardest by the earthquake and giant waves that crashed ashore Dec. 26. Volunteers hurled sacks of rice and instant noodles into trucks as U.S. helicopters loaded with other supplies buzzed overhead en route to isolated communities.

A new potential danger emerged, this time to the American and Australian military teams assisting the tsunami survivors. A radical Islamic group once headed by an Al Qaeda-linked terror chief set up a relief camp in Sumatra. The militants, known for attacking Christians on Indonesia's far-flung islands, insisted they would not interfere with foreign troops — so long as they kept to humanitarian operations.

In the Indonesian capital, Jakarta, world leaders discussed how to transform one of the largest aid packages ever assembled — nearly $4 billion in pledges — into food for the hungry and shelter for the homeless. U.N. Secretary-General Kofi Annan (search) urged nations to come forward immediately with the billions they've promised and to break with past practices of promising much and delivering little.

"The disaster was so brutal, so quick, and so far-reaching, that we are still struggling to comprehend it," Annan said. "We will never know the exact magnitude of how many men, women and children perished on 26 December."

On Friday, Annan traveled to Indonesia's tsunami-ravaged Aceh province to witness the devastation firsthand and the U.N. relief effort that is channeling relief. He was scheduled to take a helicopter ride over the devastated west coast of Sumatra island and then drive through the provincial capital, Banda Aceh.

Australia leads the world with a total aid pledge of $810 million, followed by Germany, Japan and the United States.

Secretary of State Colin Powell (search) said America would take a wait-and-see attitude before pledging more cash. "These are not insignificant numbers," Powell told reporters.

Japan hinted it might offer more help for those hit in the disaster that ravaged 11 countries — including Sri Lanka, India and Thailand. Foreign Minister Nobutaka Machimura said the $500 million Japan already pledged "was on the small side."

The World Health Organization (search) said that if basic needs — particularly access to safe drinking water — were not restored by week's end, infectious diseases could kill tens of thousands. The confirmed death toll stands at 140,000, and senior U.N. officials have predicted it will reach more than 150,000.

U.N. humanitarian chief Jan Egeland (search) said hundreds of thousands of survivors who fled the coast in Sumatra could be living in around 200 makeshift camps in the forests and the hills. Until they are interviewed about missing friends and relatives, he said, the true death toll would not be known.

"I think we have to be aware that very, very many of the victims have been swept away and many, many will not reappear. The 150,000 dead figure is a very low figure. It will be much bigger," Egeland said.

Epidemics could claim many more lives.

"We now estimate that as many as 150,000 people are at extreme risk if a major disease outbreak in the affected areas occurs," said WHO Director-General Dr. Lee Jong-wook.

For the moment, though, the threat of an outbreak of waterborne disease is being held in check by medical aid flooding into the region, U.N. officials said. While there are cases of diarrhea, respiratory and skin diseases and mental trauma, there have been no major outbreaks of disease in Sumatra's devastated Aceh region, the U.N. health agency said.

But officials are concerned that unless this aid is sustained, the system could collapse. Getting water purification tablets to survivors and building rudimentary toilets remain the focus of efforts to fend off disease.

The U.N. health organization is warning that hygiene in many of the hundreds of refugee camps that have sprung up around the region is the biggest disease concern. An estimated 3 million to 5 million people are living in refugee camps across the tsunami-stricken area.

Leaders at the summit also discussed a tsunami warning system — like one in place in the Pacific. The closing declaration called for the warning system to be set up, but gave few details.

The summit gave the United Nations overall control of coordinating the relief effort, but the U.N. official in charge on Sumatra complained that the U.S. military was failing to work with relief organizations.

Michael Elmquist said that while U.S. military helicopters were speeding relief supplies to isolated villages, their crews were not spending enough time on the ground to assess survivors' needs.

"They don't stop their engines. They're on the ground for five minutes," Elmquist told reporters in the provincial capital of Banda Aceh, on the northern tip of Sumatra.

The U.S. relief operation has won praise for reaching villages devastated by the earthquake and tsunami, which washed out roads and destroyed bridges. But Elmquist said he couldn't be sure what supplies the helicopters were delivering.

There was no immediate comment from the military. The Americans have been greeted by thankful villagers wherever their helicopters have landed.

Around Banda Aceh, aid workers, their faces covered with masks to try to ward off the stench of rotting corpses, picked up bodies and dumped them in trucks bound for mass graves. Many bodies are being buried without being identified or counted.

On some houses, signs say, "There are bodies;" in other places, arrows point to where corpses were buried under rubble.