President George W Bush and two of his predecessors, Bill Clinton and George Bush Senior, have called on Americans to aid the Asian tsunami's victims.
US helicopters are already playing a vital role in moving aid
The three appeared together at the White House to issue a joint appeal to private citizens and businesses in the world's richest nation.
The tsunami, triggered by an undersea earthquake off Indonesia, killed some 150,000 people across the Indian Ocean.
More than 1.8 million need food aid, and about five million are homeless.
Relief efforts have been gathering pace and in the worst-hit area, Indonesia's Aceh province, aid planes have been taking vital supplies to more remote areas.
US Secretary of State Colin Powell and Florida Governor Jeb Bush have arrived in Thailand to see the aftermath of the disaster for themselves.
They will also be going to Sri Lanka and Indonesia, the two countries most badly affected by the disaster.
In other developments:
- More than a million fishermen in Sri Lanka's north-east are said to have lost their livelihoods in the disaster
- International aid agencies seek access to people in the Andaman and Nicobar Islands, where foreigners are banned because of security concerns and to protect aboriginal tribes from outside influences
- A fishing boat docks in Malaysia carrying an Indonesian woman who was rescued far out to sea after five days in the water, clinging to an uprooted tree
- Six elephants which featured in the film Alexander are helping the clean-up in Thailand in hard-to-reach areas
- Indonesia says it is planning to set up a tsunami warning system
United Nations relief co-ordinator Jan Egeland said the international response had been "truly overwhelming", and that some of the world's poorest countries had responded with substantial assistance and even cash.
Mr Egeland said about $2bn had been pledged by governments and international agencies.
Outstanding co-operation involving very many nations was under way, he added.
He also confirmed the number of dead was about 150,000, but warned the number could "grow exponentially" as many coastal villages and towns in the Indonesian island of Sumatra had been wiped out.
Appeal to all
Introducing his two predecessors as the men who would head the US appeal, President Bush appealed to the "good heart of the American people".
President Bush (centre) appealed to the "good heart" of Americans
"I ask every American to contribute as they are able to do so," he said, adding that gifts of cash were the best way to help.
The Bush administration has already pledged $350m (£182m) in aid, in what President Bush said was an initial commitment to the relief effort.
Private donations have also flooded in, with the American Red Cross receiving $79.2m (£41.4m).
There has been criticism that the US government was slow to react to the disaster, so Monday's announcement looks like another public gesture to show that Washington is responding in significant ways, says the BBC's Nick Childs in Washington.
Washed-out roads and bridges are still preventing aid workers from reaching many of the millions of people in need in Aceh.
FOREIGN TSUNAMI VICTIMS
Germany: 60 dead
Sweden: 52 dead
Britain: 40 dead
France: 22 dead
Norway: 21 dead
Japan: 21 dead
Italy: 18 dead
Switzerland: 16 dead
US: 15 dead
Australia: 12 dead
South Korea: 11 dead
Figures include those feared dead but not all unaccounted for.
Sources: Reuters, AP
There has been little evidence of co-ordination between the hundreds of local charities and government departments trying to help the survivors, says the BBC's Jonathan Head in the regional capital, Banda Aceh.
Some camps get deliveries of aid they do not need, while others have been given almost nothing, our correspondent says.
After making their first visits to the camps some international agencies have expressed shock at the poor sanitation and the lack of leadership there.
A team from the International Federation of Red Cross and Red Crescent Societies has just returned from the Indonesian town of Mulabo.
According to the Red Cross, 40,000 people - about 80% of the population - there may have been killed by the giant waves. It says the survivors are walking round in a daze, and are in desperate need of help.
Military aircraft from Indonesia, the US, Australia, and Malaysia have been taking medicine, water and food from Banda Aceh to other areas in need.
There have been frantic scenes as crowds of desperate survivors scramble to retrieve air-dropped supplies.
Aid agencies say it could be several more days before all districts are reached.
An advance party of 42 US marines has also flown into Sri Lanka to prepare the ground for an eventual deployment of up to 1,500.