JAKARTA, Indonesia - World leaders pledged today to work together to help tsunami-shattered regions recover from the worst natural disaster in living memory, saying it was a race against time to get aid to survivors before they succumb to disease.

U.N. Secretary General Kofi Annan urged nations to immediately come forward with the billions in aid they've promised, amid warnings that another 150,000 people may die from disease - more than doubling the confirmed toll of 140,000.

World leaders gathered for a day in Indonesia, hardest hit by the Dec. 26 disaster, to figure out the best way to speed aid to victims. While about $4 billion has been pledged worldwide, the United Nations has warned some of the promises might not be honored as in previous disasters.

Earlier today, Secretary of State Colin Powell said that tsunami relief efforts by the United States, India, Japan and Australia - a core group formed to coordinate their initial response - will now be merged into U.N.-led operations.

"The core group helped to catalyze the international response. Now having served its purpose, it will fold itself into the broader coordination efforts of the United Nations," Powell said in remarks presented at the summit.

As the conference ended, world leaders issued a declaration pledging to work together to help the shattered region recover and set up a tsunami warning system to ensure that next time such a wave is generated coastal communities will have time to flee to higher ground.

"This unprecedented devastation needs unprecedented global response in assisting the national governments to cope with such disaster," the declaration said.

The relief and reconstruction could take up to a decade, the leaders said.

Annan urged nations to channel $1.7 billion of the funds to the United Nations for relief, including $977 million for emergency aid.

"Whole communities have disappeared," Annan said. "Millions in Asia, Africa, and even in far away countries, are suffering unimaginable trauma and psychological wounds that will take a long time to heal. Families have been torn apart.

"The disaster was so brutal, so quick, and so far-reaching, that we are still struggling to comprehend it," Annan added, stressing the need for donor "pledges to be converted into cash quickly ... It is a race against time."

The U.N. chief said the number killed across Asia and Africa would likely exceed 150,000, but the exact figure would never be known. The World Health Organization warned the toll could double if aid doesn't reach survivors soon.

"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.

A tsunami warning system - like the one already in place in the Pacific - should be established in the Indian Ocean as quickly as possible, the leaders agreed.

Japan planned to offer technical expertise to set up the warning system. The country has one of the world's most advanced networks of fiber-optic sensors, which can warn of deadly tsunami within two minutes of a quake.

"No longer must we leave ourselves so vulnerable and so exposed," Thai Foreign Minister Surakiart Sathirathaim said. "It is well-proven that 10 minutes advance warning can save hundreds of lives."

Countries around the world have pledged about $4 billion in addition to sending troops and teams of medics to the stricken regions. But there were fears much of the pledged money would not materialize.

Just more than a year ago, donors promised Iran more than $1 billion in relief after an earthquake killed 26,000 people there. Iranian officials say only $17.5 million has been sent.

As the delegates met in Jakarta, doctors warned of a looming catastrophe from ever-more serious health woes, including gangrenous wounds that require amputations, children with diarrhea and pneumonia caused by dirty water.

Indonesia was the worst affected by the earthquake and tsunami on Dec. 26 that ravaged 11 countries - including Sri Lanka, India and Thailand. The high ocean waves reached as far as eastern Africa.