U.S. President George Bush admitted Saturday that he signed an order authorizing the National Security Agency to carry out an eavesdropping program in the United States.

Bush called the program "crucial to our national security," adding the order had been used more than 30 times since the Sept. 11 terror attacks.

"This authorization is a vital tool in our war against the terrorists. It is critical to saving American lives," he said in a radio address delivered live from the White House's Roosevelt Room.

"The American people expect me to do everything in my power, under our laws and Constitution, to protect them and their civil liberties and that is exactly what I will continue to do as long as I am president of the United States."

On Friday, the New York Times ran a story claiming that Bush had authorized a program allowing the NSA to eavesdrop on Americans who were communicating with individuals overseas to determine if they had terrorist ties.

Appearing angry at times, Bush said Saturday that media reports of the highly classified program have tipped off enemies of the United States.

"As a result our enemies have learned information they should not have, and the unauthorized disclosure of this effort damages our national security and puts our citizens at risk," he said.

In a statement released earlier, Bill Keller, the Times' executive editor, said the newspaper postponed publishing the article for a year while it considered the impact of such information on national security issues.

However, the Times decided to go ahead with the report after determining it could be written without jeopardizing intelligence operations. Keller also emphasized that information about many NSA eavesdropping operations is part of the public record.

The New York Times story prompted legislators to call for a congressional inquiry into whether monitoring by the highly-secretive NSA violated civil liberties.

"There is no doubt that this is inappropriate," declared Republican Senator Arlen Specter of Pennsylvania, chairman of the Senate judiciary committee.

Defending the program today, Bush said that authorities only use the order "to intercept the international communications of people with known links to Al Qaeda and related terrorist organizations."

He added that the Justice Department, the White House counsel and others review the program every 45 days.

Program will continue

Bush also said the program would continue. "I intend to do so for as long as our nation faces a continuing threat from al Qaeda and related groups," he said.

While the NSA can eavesdrop on worldwide communications, it is barred from domestic spying. However, it can get permission to do so from a special court called the Foreign Intelligence Surveillance Act Court.

But the New York Times said the NSA has monitored international telephone calls and email messages of hundreds, perhaps thousands, of people within the United States without those warrants.

Former counter-terrorism official Richard Clarke told ABC News that normally, the NSA could monitor a conversation between Afghanistan and the United States, but the transcript could only reflect what the Afghanistan participant said.

"If, for example, the person overseas said, 'what are you going to attack in the United States?', we would get the question, but we wouldn't get the answer from the person in the United States under previous policy," he said.

Bush claims some of the taps have foiled terrorist attacks, including one proposed for the Brooklyn Bridge -- but critics say there's no way to verify that.

One critic, Rep. Dennis Kucinich (D-Ohio), said: "We don't have government looking over our shoulder everywhere, trying to see what we're doing or listening into our phone calls. That's not what America is about."

With a report from CTV's Graham Richardson