FBI director Robert Mueller has offered testimony that appears to contradict sworn statements by embattled US Attorney General Alberto Gonzales.
US Attorney General Alberto Gonzales is under intense pressure
Mr Mueller told Congress that top administration officials had differed over a wiretapping programme in 2004.
In sworn testimony on Tuesday, Mr Gonzales denied that there had been disagreements over the programme.
Meanwhile presidential aides have been summoned to testify on Mr Gonzales' sacking of eight federal prosecutors.
Top strategist Karl Rove and White House deputy political director Scott Jennings have been subpoenaed by Democratic senators over the controversial dismissals.
The Democrats have also announced they will seek a perjury investigation into Mr Gonzales in connection with the same case.
The administration says the congressional moves are politically motivated and denies Mr Mueller's testimony contradicts that of Mr Gonzales.
The latest dispute centres on the Terrorist Surveillance Program (TSP) - which allowed the National Security Agency to eavesdrop on phone calls made by terror suspects.
In May a former deputy attorney general testified that he and other Justice Department officials had threatened to resign in 2004 over legal objections.
This was denied by Mr Gonzales, who was then serving as White House Counsel.
In a statement to the Senate on Tuesday, he said meetings in March 2004, including one at the hospital bedside of the then Attorney General John Ashcroft, involved different intelligence activity.
However during testimony to the House Judiciary Committee on Thursday, Mr Mueller was asked if the discussion in the hospital room had been on the TSP.
He answered that he had arrived at the meeting after the presidential aides had left, but that he understood "the discussion was on a national NSA programme that has been much discussed, yes".
Later a statement by the Justice Department reaffirmed that Mr Gonzales' account had been truthful and that "confusion is inevitable when complicated classified activities are discussed".
The row over last year's firing of the prosecutors has been rumbling for several months.
Members of Congress, which has been controlled by the Democrats since the beginning of the year following mid-term elections, have been pitted against senior figures in the Bush administration.
Mr Gonzales, the nation's top law-enforcement officer and an old friend of Mr Bush, has faced repeated calls to resign over the affair.
Opponents say he fired the eight attorneys for political reasons and later lied about the reason for their dismissal.
Mr Gonzales says he did nothing wrong. He points out that US attorneys serve at the will of the president, who can dismiss them at any time.
He has testified before committees in both houses of Congress, but senators now say he lied under oath.
Democrats say that Mr Rove will be able to shed light on the dismissals. They say he knew of discussions about firing the attorneys nearly two years before the axe fell on them.
President George W Bush has previously offered to let Mr Rove and other aides speak privately to some members of Congress, but he has firmly rejected the demand for public testimony under oath.