"It's seems logical to me that if we know there's a phone number associated with al Qaeda or an al Qaeda affiliate and they're making phone calls, it makes sense to find out why," Mr. Bush said. "They attacked us before, they'll attack us again."
Mr. Bush spoke to reporters at Brooke Army Medical Center where he was visiting wounded troops. He said the leak of information about the secret order to eavesdrop on Americans with suspected ties to terrorists causes "great harm to the nation."
Asked how he responds to Americans worried about violations of their privacy, he responded, "If somebody from al Qaeda is calling you, we'd like to know why."
CBS News chief White House correspondent John Roberts reports in answer to criticism about possible civil liberty violations, Mr. Bush emphases the program is limited in scope.
"This is a limited program designed to prevent attacks on the United States of America and, I repeat, limited," he said. "I think most Americans understand the need to find out what the enemy's thinking."
The Justice Department has begun investigating the leak to The New York Times that resulted in a story last month about warrantless surveillance conducted by the National Security Agency since the terrorist attacks of Sept. 11, 2001.
Mr. Bush, who called the program "vital and necessary," dodged a question about whether he was aware of any resistance to the program at high levels of his administration and how that might have influenced his decision to approve it.
He said the program has been reviewed by Justice Department officials and members of Congress and that it continues to be reviewed.
"The NSA program is one that listens to a few numbers called from the outside of the United States of known al Qaeda or affiliated people," he said, adding that he believes that he is acting within the law.
"The fact that somebody leaked this program causes great harm to the United States," he said. "There's an enemy out there."
Many Democrats and some Republicans in Congress have questioned whether Mr. Bush's actions have gone beyond the constitutional powers and congressional resolution he has cited in defense of his actions authorizing the secret program.
Sen. Arlen Specter, the Pennsylvania Republican who chairs the Judiciary Committee, has called for hearings into the program.
In 1978 Congress established a secret court to handle requests for surveillance and to issue warrants — a system the Bush-authorized program bypassed.
The president was asked whether he misled the American people in 2004 when, during an event promoting the Patriot Act, he said that any wiretapping required a court order and that nothing had changed. He made the statement more than two years after he approved the NSA program.
"I was talking about roving wiretaps, I believe, involving the Patriot Act," Mr. Bush said. "This is different from the NSA program."