The disclosure follows angry demands by lawmakers earlier in the day for congressional inquiries into whether the monitoring by the highly secretive National Security Agency violated civil liberties.
"There is no doubt that this is inappropriate," declared Republican Sen. Arlen Specter of Pennsylvania, chairman of the Senate Judiciary Committee. He promised hearings early next year.
Bush on Friday refused to discuss whether he had authorized such domestic spying without obtaining warrants from a court, saying that to comment would tie his hands in fighting terrorists.
In a broad defense of the program put forward hours later, however, a senior intelligence official told The Associated Press that the eavesdropping was narrowly designed to go after possible terrorist threats in the United States.
The official said that, since October 2001, the program has been renewed more than three dozen times. Each time, the White House counsel and the attorney general certified the lawfulness of the program, the official said. Bush then signed the authorizations.