"An enormous effort will be required on the part of many to cope with the human and physical destruction," Greenspan said in testimony to the Senate Banking Committee.
In his speech to Congress, set for 9 p.m. EDT, Bush will make the case against No. 1 suspect Osama bin Laden and his al-Qaida network, officials said.
With the speech in the Capitol taking place under exceptionally tight security, Fleischer said that Vice President Dick Cheney will not attend but will remain in a secret, secure location
The decision is a recognition of "the continuation of important government issues" should terrorists strike again, Fleischer said.
British Prime Minister Tony Blair will attend, seated with first lady Laura Bush, Fleischer said.
In the Afghan capital of Kabul, Islamic clerics Thursday urged bin Laden to leave Afghanistan voluntarily, but set no deadline for him to decide, according to the news agency of the ruling Taliban militia.
The clerics said they are prepared to call for a holy war against the United States if U.S. troops attack Afghanistan in an attempt to capture him and members of his al-Qaida terrorist organization.
Conveying Bush's rejection, Fleischer said, "This is about much more than any one man being allowed to leave -- presumably from one safe harbor to another safe harbor, if what he's doing is voluntary."
"The president has demanded that the key figures of the al-Qaida terrorist organization, including Osama bin Laden, be turned over to responsible authorities and that the Taliban close terrorist camps in Afghanistan. The United States stands behind those demands," Fleischer said.