He then took part by video link in a meeting in Washington that included Secretary of State Colin L. Powell, the president of the World Bank, James D. Wolfensohn, and the representatives of three other nations — Australia, India and Japan — that have formed a "core group" along with the United States to lead in coordinating the relief drive.

The White House announced that Powell would tour the region to assess its needs next week along with the president's brother, Gov. Jeb Bush of Florida.

Annan said he welcomed the group's initiative because the regional powers could immediately bring essential resources to the task and "support the United Nations effort."

Speaking of the challenge ahead, Annan said, "It is so huge that no one agency or one country can deal with it alone and that we need to coordinate our efforts and pool our efforts to have maximum impact on the crisis."

"This is an unprecedented global catastrophe, and it requires an unprecedented global response."

Meanwhile, millions of water-purification tablets are being sent to the affected countries where the need for safe water is dire, UNICEF's Tobin said.

UNICEF already had large storage tanks for water in India and has moved some of them to affected areas in the south and east, said a UNICEF spokesman, Alfred Ironside. The tanks can be set up in communities and then refilled by tanker trucks, he said. Families are then given clean jerry cans to carry their own supplies.

"In the early days, a family may have to walk a mile or two inland to where water systems were not affected by flood waters," Ironside said. "The jerry cans are good for that." But he added that the system was in place mainly in India and in Sri Lanka, not in Indonesia, the scene of much of the worst devastation.

Conditions vary, he said. In parts of Indonesia, for instance, the floodwaters surged as far as two miles inland. In Sri Lanka, the waves came inland between a few hundred yards and half a mile or so.

"Not much further inland, everything is functioning," he said. That means clean water is available nearby but must be carried to the people who need it.

"A lot of homegrown solutions are happening," Ironside said. "Private donors of all kinds are driving in with bottled water, especially in Sri Lanka and India."

A team from an independent disaster-aid group, Medair, is expected to arrive Friday in the Ampara district in eastern Sri Lanka, across the island from the capital city, Colombo, said Robert Schofield, a spokesman for the group. The team was bringing medical supplies, chemicals for water purification, a doctor and a water and sanitation engineer.