"We were more generous when we were less rich, many of the rich countries," said Jan Egeland. "And it is beyond me, why are we so stingy, really ... Even Christmas time should remind many Western countries at least how rich we have become."
The U.S. has so far pledged $15 million in disaster aid.
Appearing Tuesday on ABC's "Good Morning America," Secretary of State Colin Powell said the U.S. would increase assistance to disaster-stricken Asian nations.
"We will do more. I wish that comment [by Egeland] hadn't been made," Powell said.
"We'll make an assessment as the days go by, to see what more is needed of us," he said. "It will take us awhile to make a careful assessment of what is needed ... to see what the specific needs are and then we will respond to those needs."
In a separate interview on NBC"s "Today" show, Powell said that "clearly, the United States will be a major contributor to this international effort. And, yes, it will run into the billions of dollars."
American ambassadors have released $100,000 each to India, Indonesia, the Maldives and Sri Lanka, and Powell said $4 million would be given to the Red Cross.
U.S. government specialists in disaster relief were sent to Thailand and Indonesia, and others will be spread out through the region.
Supplies of shelter, food and water cans kept in reserve in the Philippines and in Dubai will be distributed, according to Ed Fox, assistant administrator of the U.S. Agency for International Development.
Millions of people who were displaced will need shelter, food and clothing, Fox added. The $15 million U.S. contribution was an initial one, he said, issued while surveys were conducted.
The Australian government pledged $7.6 million in immediate aid.
A spokesman at U.S. Pacific Command in Hawaii said that in addition to three Navy P-3 Orion surveillance planes sent to Thailand, the military also is loading five or six Air Force C-130 cargo planes with tents, clothing, food and other humanitarian goods for delivery to Thailand.
Pacific Command also is assembling small assessment teams that will be dispatched to three countries in the region to assess how U.S. military resources can best be applied in those countries.
Pacific Command spokesman Lt. Col. William Bigelow said he was not authorized to identify the three countries, but other government officials said they were Sri Lanka, Indonesia and Thailand.
And James D. Wolfensohn, president of the World Bank, said bank teams were discussing potential assistance with the governments of the countries that suffered losses.
On Sunday, the managing director of the International Monetary Fund, Rodrigo de Rato, said the Fund "stands ready to do its part to assist these nations with appropriate support in their time of need."
The U.S. Navy said it sent three P-3 surveillance aircraft from Kadena air base on the Japanese island of Okinawa to Utaphao, Thailand, to conduct survey operations, and possibly aid in search-and-rescue efforts.
The Navy said it had no reports of damage to any of its ships or bases in the region.