JAKARTA, Indonesia - The United Nations said today that camps for up to 500,000 tsunami refugees will be built on devastated Sumatra island, while world leaders headed to Indonesia to discuss how to distribute billions of dollars in aid.

Indonesia's government has started breaking ground on four camps around Banda Aceh, the main city in northern Sumatra, where an estimated 1 million people were left homeless by the tsunami. The United Nations plans to provide tents and equipment for up to 500,000, said Michael Elmquist, who heads the U.N. relief effort in Aceh.

The existing camps are overcrowded and lack facilities. Indonesian authorities have agreed the new camps will have clean drinking water and latrines, Elmquist said.

"The camps that are here have been improvised by the people themselves," he said. "But these are definitely not according to our standards. The sanitation is totally insufficient."

Meanwhile, U.N. Secretary-General Kofi Annan, Secretary of State Colin Powell, Australian Prime Minister John Howard, Chinese Prime Minister Wen Jiabao and Japanese Prime Minister Junichiro Koizumi were among the officials expected in the Indonesian capital, Jakarta, for a summit on tsunami relief that begins Thursday.

Already about $2 billion has been pledged from around the world to help the millions of victims rebuild from the Dec. 26 tsunami. There have been 140,000 confirmed deaths from the 9.0 magnitude earthquake that rattled the ocean floor and sent massive waves crashing across beachfront communities from southern Asia to Africa on Dec. 26.

"I've been in war and I've been through a number of hurricanes, tornados and other relief operations, but I've never seen anything like this," Powell said today after flying over shattered apocalyptic landscapes on Indonesia's Sumatra island.

The United Nations has performed rapid assessments in countries hit by the tsunami, and Thursday's meeting aims to get donors to commit to specific aid and reconstruction projects, said Bo Asplund, U.N. representative in Indonesia.

Topping the list of demands is Indonesia, Asplund said, with some $450 million required under a U.N. appeal for the country that suffered at least 94,200 deaths.

Bringing together representatives of all the affected countries will allow aid officials to get commitments for relief for at least the next six months, said Elmquist. The countries also hope to prevent future disasters by creating a warning system for tsunamis in the Indian Ocean.

Another issue on the agenda will be possible debt relief to affected countries, many of which are developing nations that rely on international assistance. Britain's top treasury official has said the world's richest nations are likely to freeze debt repayments and may even forgive debts to countries hit by the tsunami.

Marine helicopters today buzzed into Medan, Indonesia, not to bring food in, but to take supplies away.

Aid has flooded into Medan and the supplies had been stacked in disorganized piles near a warehouse at the city's airport, an overwhelming amount beyond what was immediately needed in the area.

Desperate to get the supplies to hard-to-reach areas on Sumatra's west coast, CH-46 helicopters from the USS Bonhomme Richard launched an airlift operation Wednesday morning to bring the aid back to their ship.

Meanwhile, the fragility of relief efforts was underscored by the temporary closure Tuesday of the main, overstretched airport in Sumatra. Today, a load of aid supplies fell from a U.S. helicopter over the island's city of Medan, hitting a shopping mall. No one was hurt.

Pilots were ferrying survivors from hard-hit towns and villages in the region to medical help. But that created a new challenge for relief workers: bottlenecks at overcrowded hospitals.

About a dozen people lay on stretchers on the sidewalk outside Fakina Hospital in Banda Aceh. Many of the hospital's rooms had no power. Walls were flecked with blood and doctors had run out of stands for intravenous fluid bags, hanging them from cords strung across the ceiling.

"It's heartbreaking," said Leslie Ansag of Everett, Wash., a Navy medic from the American aircraft carrier USS Abraham Lincoln, which was off Sumatra to help the rescue effort.

In Thailand, where more than 2,200 foreign tourists were among 5,000 killed, police said they were searching for a 12-year-old Swedish boy last seen leaving a hospital with an unknown man the day after the tsunami hit. Authorities said they could not confirm media reports that Kristian Walker had been kidnapped; a German sought in the boy's disappearance turned himself in and has been cleared, police said today.

Police and U.N. officials have expressed fears that trafficking gangs will exploit the chaos of the disaster to abduct children and sell them into forced labor or even sexual slavery.

In Aceh, U.N. officials said broken bones and infected wounds are the biggest health problem facing staff in overflowing hospitals.

UNICEF director Carol Bellamy and World Health Organization Director-General Dr. Lee Jong-wook toured the provincial capital, Banda Aceh, today, visiting hospitals and makeshift clinics tending the thousands of people injured in the Dec. 26 earthquake and tsunami.

Staff at the hospitals said that many people had infected wounds sustained in the disaster, some of which were turning gangrenous, forcing surgeons to amputate limbs.

Europeans were to honor the disaster's 140,000 victims today, observing a European Union call for three minutes of silence at noon.

Flags were lowered to half-staff, stock exchanges were to stop trading and public transport was to come to a temporary standstill in many capitals across the 25-nation bloc in silent tribute.

In Thailand, rescue workers freed a humpback dolphin from a small lagoon where the tsunami dumped it, returning it to the Andaman Sea.

The dolphin, spotted Monday about a half a mile from the beach by a man searching for his missing wife, had become a symbol of hope amid the death and destruction.