President Clinton has again denied he had a sexual relationship with a former White House aide, Ms Monica Lewinsky, and promised full co-operation with the investigation now under way by independent counsel Mr Kenneth Starr.
There are indications that Mr Clinton is anxious to give his side of the story in a major press conference before his State of the Union address next Tuesday.
The President's staff and supporters were dismayed at what was seen as weak, legalistic denials of the allegations on Wednesday, when the first reports appeared. A more robust defence is now seen as imperative if Mr Clinton is to recover from this latest allegation, following so soon after his six-hour deposition last Saturday to lawyers for Ms Paula Jones, who has accused him of sexual harassment.
Mr Clinton has also insisted to reporters that he did not urge Ms Lewinsky to commit perjury when she was asked to testify about her relationship by lawyers for Ms Jones. Republican politicians and legal experts say that if such a charge were proved against him, he could face impeachment and removal from the presidency.
Mr Starr is demanding full records from the White House concerning Ms Lewinsky, who has been recorded on tapes discussing an affair with Mr Clinton. But she has also sworn an affidavit denying such an affair.
Mr Clinton is struggling to continue with his presidential duties while the latest allegations are dominating Washington, and overshadowing coverage of the Pope's visit to Cuba.
Mr Clinton was questioned by reporters about Ms Lewinsky as he began his meeting in the Oval Office with the Palestinian President, Mr Yasser Arafat.
Mr Clinton said: "The allegations are false and I would never ask anybody to do anything rather than tell the truth." He said the American people have a "right to know" the truth of the matter and he promised to provide information as quickly as possible.
Mr Starr told the media he would pursue rapidly his investigation into the allegations against the President and his adviser, Mr Vernon Jordan, both of whom are alleged to have urged Ms Lewinsky to deny she had an affair with him.
She is due to testify orally to Ms Jones's lawyers today about her contacts with Mr Clinton. Sources say that she will plead the Fifth Amendment to avoid incriminating herself in a possible perjury case.
The major newspapers agree in editorial comment that this is the most serious crisis facing Mr Clinton since he became President in 1993.
The New York Times said: "It is not the legality of anyone's sexual behaviour that is at issue here. The legal questions before Mr Starr are obstructions of justice, perjury and suborning of perjury.
"If Mr Clinton's denials do not hold up, his presidency will be thrown into a disabling political crisis."
The Washington Post said the allegations against President Clinton are "allegations of extremely serious crimes."
A CNN/USA Today poll showed Americans would be more concerned that the President tried to obstruct justice than if he had an affair with Ms Lewinsky. Some 72 per cent say they would find this relevant to his job as President, while 52 per cent say that an extra-marital affair would not be relevant.