A US federal judge has ruled that President Bill Clinton committed "a criminal violation of the Privacy Act" by releasing personal letters to undermine the credibility of one of his accusers during the Monica Lewinsky saga.

US District Judge Royce Lamberth concluded the Democratic president and three top lawyers in the White House disregarded an earlier court ruling in making public letters from a White House employee, Kathleen Willey.

President Clinton immediately disputed the decision, saying he reluctantly released the letters two years ago because "it was the only way I knew to refute allegations" by Ms Willey of an unwanted sexual advance.

Initially, the ruling - made in a case in which a right-wing lobby group, Judicial Watch, is trying to get documents to prove that the White House attacked its opponents - simply requires the president's lawyers to answer questions they earlier rejected about the decision to release the letters.

But it could open the door for an eventual lawsuit against President Clinton.

A violation of the Privacy Act is a misdemeanour which carries no possible prison sentence.

The president said he never even considered the Privacy Act when he made the decision to release the letters.

The president said he "decided to do it reluctantly only because it was the only way I knew to refute allegations that were made against me that were untrue".

He also took a swipe at Judge Lamberth, noting the Republican appointee "somehow acquired a significant percentage of the cases involving the White House. That's an interesting story."