The initial US grant of $35 million in the aftermath of the tsunami disaster was "only the beginning of our aid," he said Wednesday at his ranch in Texas, where he is holidaying.
Bush also spoke to the leaders of India, Sri Lanka, Thailand and Indonesia on Wednesday, and said criticism that rich nations were not providing enough aid were not true.
"There will be an assessment of the damage so that the next tranche of the relief will be spent wisely. ... Our government is fully prepared to continue to provide assistance and help," he said.
"We will stand with them as they start to rebuild their communities."
He also pledged a multi-pronged US response, involving US military manpower, damage surveillance teams and long-term rebuilding assistance.
"This has been a terrible disaster. It is beyond our comprehension," he declared.