Chuck Hagel and Olympia Snowe, Republican Senators, joined Carl Levin, Dianne Feinstein and Ron Wyden, Democratic Senators on Tuesday in calling for a joint investigation by the Senate Intelligence and Judiciary Committees into whether the government eavesdropped "without appropriate legal authority".
Several Republican and Democratic lawmakers have already backed a plan for a congressional hearing into the programme, first revealed by The New York Times last week.
But the White House on Tuesday brushed aside calls for congressional hearings.
"This is still a highly classified programme and there are details that it's important not be disclosed," Scott McClellan, White House spokesman, said.
Bush, Dick Cheney, the US vice-president, and other senior administration officials have defended the policy of authorising - without court orders - eavesdropping on international phone calls and e-mails by Americans suspected of links to terrorism.
They argue it was legal and provided the agility - beyond a 1978 law allowing court-warranted eavesdropping - to help defend the country after the 11 September, 2001 attacks.
The White House also sought to play down the impact on civil liberties, arguing the programme was narrow in scope and that key congressional leaders were "briefed in the appropriate way" about the programme.
Cheney insists that the spying
programme was justified
Senior Democrats said those briefings left out key details and that Congress was prevented from exercising its oversight authority because the information was classified.
Cheney, speaking to reporters during an overseas trip, forcefully defended the eavesdropping programme as necessary to the nation's defence.
"The president and I believe very deeply that there is a hell of a threat," he said, adding this obliged them to "do everything in our power to defeat the terrorists".
"And I don't think that there is anything improper or inappropriate in that and my guess is that the vast majority of the American people support that, support what we're doing, believe we ought to be doing it," he said.
"So there's a backlash pending, I think the backlash is going to be against those who are suggesting somehow we shouldn't take these steps in order to defend the country," he said, speaking on a plane to Oman from Pakistan.
The eavesdropping programme is the latest in a series of administration policies in Bush's declared war on terrorism that have prompted questions over whether the line has been crossed between protecting the public and protecting civil rights.
The senators calling for an investigation demanded detailed information on the programme, including on its legality.
"It is critical that Congress determine, as quickly as possible, exactly what collection activities were authorised, what were actually undertaken, how many names and numbers were involved over what period, and what was the asserted legal authority for such activities. In sum, we must determine the facts," they said in a joint letter.