The Justice Department asked a federal appeals court to throw out a lower court decision that said the Bush administration's surveillance program is unconstitutional.
President George W. Bush has said the program is needed to detect terrorists. Opponents argue it oversteps US constitutional boundaries on free speech, privacy and executive powers.
Government lawyers said in pleadings filed Friday with the 6th US Circuit Court of Appeals in Cincinnati that the surveillance program "is necessary to protect the nation from an ongoing national security threat of the highest order and is vital to waging and winning the ongoing armed conflict."
US District Judge Anna Diggs Taylor in Detroit ruled August 17 that the program violates the rights to free speech and privacy and the separation of powers.