Getting aid to the millions of victims of the tsunami is a race against time, U.N. Secretary-General Kofi Annan (search) said Thursday, the start of an emergency summit to figure out the best way to spend nearly $4 billion that has been pledged worldwide.

He told leaders from around the world who have gathered in Jakarta for the meeting that the death toll from the Dec. 26 tragedy would likely exceed 150,000, though the exact figure would never be known.

"Whole communities have disappeared," he 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.

Indonesian President Susilo Bambang Yudhoyono (search), the summit's host and the leader of the hardest-hit country, described the calamity as "the most destructive natural disaster in living memory."

"Our response to this unprecedented catastrophe must be equally unprecedented," he said at the start of the meeting.

The leaders at the one-day conference — including Chinese Prime Minister Wen Jiabao, Japanese Prime Minister Junichiro Koizumi and U.S. Secretary of State Colin Powell (search) — are discussing the best way to get nearly $4 billion that has been pledged worldwide to the victims.

"There are daunting logistical constraints. But they are not insurmountable. It is a race against time," Annan said.

He said the world must set up a tsunami-warning system to ensure such a tragedy is not repeated. Such a system is already in place in the Pacific Ocean, but not the Indian.

"Although we were powerless to stop the tsunami, together we have the power to stop those next waves," he said.

Pledges of aid rushed in on the eve of Thursday's conference.

Australia raised its total aid pledge to $810 million, the largest government contribution, topping Germany's $660 million, followed by Japan and the United States.

The United States was the first to raise the aid race stakes last week by pledging $350 million. It's now fourth on the donor list and has sent in two aircraft carrier groups and thousands of troops. Japan promised $500 million last week.

The United Nations warned that some pledges might not be honored.

A little over 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.

Amid the apparent international largesse, aid agency Oxfam (search) called Wednesday on donors to provide relief as grants — not loans — over at least a five-year period.

"We must ensure we don't repeat mistakes of previous humanitarian crises in Afghanistan, Liberia and elsewhere where donors have either failed to deliver the aid quickly enough or at all, or delivered aid at the expense of other disasters," Oxfam Director Barbara Stocking said in a statement.

Hundreds of heavily armed police and troops ringed the plush Jakarta conference center hosting the summit.

Security already is tight throughout Jakarta, where Al Qaeda-linked terrorists have been blamed for blowing up a hotel and the Australian embassy recently, as well as the 2002 Bali bombings, which killed 202 people, mostly foreigners.