US President George W Bush has strongly defended his domestic spying programme, which involves eavesdropping on people with suspected ties to terror groups.
Mr Bush said the operation involved telephone calls from outside the US by people associated with al-Qaeda
Mr Bush, speaking on a visit to San Antonio, Texas, said the programme was vital and necessary to protect the US.
The US Justice Department has opened an inquiry into how information about the programme was leaked to a newspaper.
Last month, the New York Times reported on how the National Security Agency had been spying in the US without warrants.
Mr Bush later admitted he authorised the surveillance activities after the 9/11 attacks.
On Sunday, the president told reporters after visiting wounded troops at Brooke Army Medical Center that the surveillance involved telephone calls from outside the US by people associated with al-Qaeda.
Questioned by reporters, Mr Bush said he was conscious of people's civil liberties and the violation of their privacy - but added: "If somebody from al-Qaeda is calling you, we'd like to know why."
"We're at war," the president said.
He called the operation legal as well as vital to thwarting terrorist attacks, and contended the leak making it public had caused "great harm to the nation".
"This is a limited program designed to prevent attacks on the United States of America and, I repeat, limited," Mr Bush said.
But senators from both the Republican and opposition Democrat parties have expressed concern about the "inappropriate" and "Big Brother" monitoring programme.
The Bush administration has argued that eavesdropping is legal and that congressional leaders were told of the programme.
But, on Sunday, the New York Times reported that the top deputy to then-Attorney General John Ashcroft refused to approve key parts of the operation in 2004.
The newspaper reported that James Comey was concerned about the legality of the NSA programme and refused to extend it.