U.S. President George W. Bush walked through the rubble where the World Trade Center used to stand Friday, praising rescue workers for their efforts.

"The nation sends its love and compassion to everybody who's here," he shouted into a megaphone.

"Thank you for your hard work. Thank you for making the nation proud. And may God bless America."

The spirits of many tired searchers appeared to lift when the president arrived and started shaking hands.

"I want you all to know that America, today, is on bended knee in prayer for the people whose lives were lost here, for the workers who work here, for the families who mourn," Bush said.

He put his arm around a firefighter as he spoke to the crowd. When some people shouted that they couldn't hear him, Bush responded with assurances: "I can hear you. The rest of the world hears you. And the people who knocked these buildings down will hear all of us soon."

The rescue workers, gathered among the tonnes of broken concrete and twisted steel, cheered and chanted "U.S.A." after the president finished his brief remarks.

Thousands of people are believed to have been killed when the skyscraper was destroyed by terrorists Tuesday morning. Fewer than a dozen survivors have been pulled out since Wednesday.

Bush flew over the smouldering wreckage in a helicopter, escorted by fighter jets, before touring the devastation up close. Earlier in the day, he attended a special memorial service in Washington.