Indonesian President Susilo Bambang Yudhoyono, meanwhile, toured Aceh province to assess recovery efforts. About a third of the 320,000 residents of Aceh's capital, Banda Aceh, are dead or missing.
Bush and Clinton were asked by current President George W. Bush to lead the U.S. effort to provide private aid to hundreds of thousands of tsunami victims. They also plan to visit Indonesia, Sri Lanka and the Maldives.
The ex-presidents stood in intense tropical heat in the shattered Thai village of Ban Nam Khem as children who lost family members in the tsunami presented them with drawings, one showing a giant wave and a rescue helicopter and the other of floodwaters sweeping away people, cars and boats. Bush and Clinton later visited a memorial wall honoring foreign tourists who died, and they then dined with Thai Prime Minister Thaksin Shinawatra.
"I don't think there's ever been a tragedy that affected the heartbeat of the American people as much as this tsunami has done," the senior Bush said. "I don't think you can put a limit on it. It's so devastating. They're still finding wreckage, still actually some bodies being recovered."
The official tsunami death toll in Asia ranges from 169,070 to 178,118. The number of missing is believed to be as high as 128,426, with most presumed dead.
After arriving on the Thai resort island of Phuket on Saturday, Bush and Clinton made their way by U.S. military helicopter and then motorcade to Ban Nam Khem.
A crowd of several hundred villagers greeted them from behind barriers, and a group of Thai school children in red caps and white shirts waved paper American flags. One banner in the crowd read: "Bill, let's talk please."
Clinton said an estimated one-third of American households have contributed to tsunami relief. He said governments and private individuals had committed $7 billion to tsunami relief in Asia, and another $4 billion was needed for a reconstruction process that could take two years.
Thaksin said Thais were "very touched and grateful" for American support.
With foreign aid workers having poured into tsunami-hit areas — especially Aceh where rebels have waged a separatist war since 1976 — fears have been raised for their safety.
Australian Foreign Minister Alexander Downer said the government had received new information "concerning possible terrorist planning for attacks against foreigners involved in relief efforts in Aceh or other areas of northern Sumatra" island in Indonesia.
The government revised its travel advice for Indonesia, warning Australians could endanger themselves by visiting northern Sumatra island.
However, Yudhoyono, Indonesia's president, said while touring Aceh on Saturday that he had checked with local police and military commanders about the threat.
"There is no evidence that there has been a threat against the aid workers," he said. "Of course, Indonesia is responsible to provide security to protect all workers."
It was Yudhoyono's sixth visit to the region since assuming office in October.
Yudhoyono said that upcoming peace negotiations with separatist rebels from tsunami-hit Aceh province would focus on the government's plan to end the conflict by granting the region self-government within Indonesia.
Yudhoyono said that Indonesian negotiators had been dispatched to Helsinki, Finland, after mediators confirmed that leaders of the Free Aceh Movement, also known as GAM, had agreed to hear out the government's proposal on wide-ranging autonomy for the province of 4.1 million people.
The peace process, which collapsed in 2003 after the military launched a major offensive, was restarted last month under the auspices of the Crisis Management Initiative of former Finnish President Martti Ahtisaari. The two sides are scheduled to meet again on Monday in Helsinki.
"After getting a report from Helsinki that GAM agrees to stick to the agenda ... toward the acceptance of special autonomy and topics (arising) from it," the Cabinet agreed to continue negotiations, Yudhoyono told reporters.
On Friday, U.N. officials said about 790,000 tsunami survivors in Aceh are still unable to feed themselves and will need food rations for many more months.
Meanwhile, Japanese researchers are developing a faster, more precise tsunami-alert system that would directly monitor the earthquake-triggered waves as they sped toward shore, media reports said.
Japan's warning system — one of the world's most advanced — can issue an alert for a deadly tsunami within minutes of an undersea quake. But false alarms are common because the system's warnings are estimates based on a quake's magnitude and epicenter — not on actual wave measurements.
Researchers at Akita University in northern Japan and a state-funded Japanese institute want to remove the guesswork with a system that measures the rise and fall of the ocean's surface with shortwave radar, the Nihon Keizai financial newspaper and other dailies said.