A top U.S. aid official said Beijing had not yet requested assistance.

The United States has search-and-rescue teams on standby in Virginia and California, said Ky Luu, the director of foreign disaster assistance for the U.S. Agency for International Development. Washington is also prepared to send a disaster assistance response team if asked, he said.

But the quickest way to help would be funding, he said.

The massive 7.9-magnitude earthquake killed more than 8,500 people in China and left hundreds trapped under rubble, state run media reported early Tuesday. About 900 students were reported buried under a collapsed school.

China's Seismological Bureau said the quake affected more than half the country's provinces and municipalities

China's government is releasing $2.89 million to respond to the disaster, China's official Xinhua news agency reported.

The U.S. ambassador is speaking to Chinese officials and once a disaster declaration is made, it will release U.S. money to help.

Luu said Beijing has good disaster-response mechanisms of its own.

"The Chinese have a strong capability of responding," he said, adding that the United States doesn't want to displace the internal expertise. "There is a 72-hour window of opportunity and it may be best to support regional teams on the ground."

President Bush said: "I am particularly saddened by the number of students and children affected by this tragedy."

China's President Hu Jintao ordered an all-out effort to help those affected, and Premier Wen Jiabao traveled to the region to direct the rescue work, Xinhua reported.

"My fellow Chinese, facing such a severe disaster, we need calm, confidence, courage and efficient organization," Wen was quoted as saying. "I believe we can certainly overcome the disaster with the public and the military working together under the leadership of the CPC Central Committee and the government."