President George W Bush today acknowledged he signed a secret order after the September 11th, 2001, attacks to allow the surveillance of people in the United States.
In a rare live radio address, Mr Bush defended the practice as a "vital tool" in defending the United States against another such attack.
The presidential order was first reported in The New York Times yesterday.
The report said the order allowed the National Security Agency to track international telephone calls and e-mails of hundreds of people without the court approval normally required for domestic spying.
The report immediately prompted concerns among both Democrats and Republican in Congress and the Bush administration initially declined to confirm it.
In the radio address, Mr Bush called for renewal of the USA Patriot Act, an anti-terror measure stalled in Congress. Some opponents of the renewal have cited the report as adding to their concerns about the need to insure that Americans' civil liberties are protected.
Mr Bush said his order was constitutional and has been carefully reviewed by legal authorities. He also criticised the leak of the information to the media.
"In the weeks following the terrorist attacks on our nation, I authorised the National Security Agency, consistent with US law and the Constitution, to intercept the international communications of people with known links to al-Qaeda and related terrorist organisations," he said.
"This is a highly classified program that is crucial to our national security," Mr Bush aid.