Key Points:

WASHINGTON - A former White House aide apologised for calling one of nine fired federal prosecutors "lazy" but refused to answer a host of questions at a congressional probe of the dismissals.

Sara Taylor, who had been White House political director until six weeks ago, testified under oath before the Senate Judiciary Committee in response to a subpoena.

But at the direction of President George W Bush, Taylor refused to answer certain questions about the administration's ouster last year of nine of the nation's 93 US attorneys.

"I will answer faithfully those questions that are appropriate for a private citizen to answer while doing my best to respect the president's directive that his staff's communications be privileged," Taylor said.

Bush, under fire on a number of fronts from the Iraq war to US immigration policy, claimed executive privilege on Monday to shield Taylor and former presidential counsel Harriet Miers from having to testify to Congress about the dismissals.

Critics charge that the firings of the prosecutors appear to have been politically motivated - perhaps even to influence investigations of Democratic or Republican lawmakers.

A House of Representatives Judiciary subcommittee said it expected Miers to testify on Thursday. But her attorney advised the panel she would not be there, drawing quick criticism.

"It is disappointing that Ms Miers has chosen to forgo this opportunity to give her account of the potential politicisation of the justice system," said the subcommittee chairwoman, Rep Linda Sanchez, a California Democrat.

US Attorney General Alberto Gonzales signed off on the firings as part of a plan that originated at the White House shortly after Bush was reelected to a second term in 2004.

Bush and Gonzales have said the dismissals were justified but mishandled. With the support of Bush, Gonzales has rejected bipartisan calls to resign.

Caught in the middle of a battle between the White House and Congress, Taylor was guarded in her testimony. But at times went on the offensive.

"I don't believe that anybody did anything wrong or improper with respect to this issue," she said at one point.

Taylor cited a letter from White House counsel Fred Fielding that advised her not to answer questions "concerning White House consideration, deliberations or communications, whether internal or external, relating to the possible dismissal or appointment of US attorneys."

She then declined to answer other questions, including whether she discussed the firings with White House political strategist Karl Rove, if she had made any suggestions and who had decided who should be fired.

But Taylor did say she had never spoken to Bush about replacing the prosecutors or attended a meeting with the president about the matter.

Taylor also responded when asked about an email she wrote this year in which she said "Bud (Cummins) is lazy -- which is why we got rid of him" as the US attorney in Arkansas.

Taylor told the committee: "I would like to take this opportunity to apologise to Mr Cummins. It was unkind and unnecessary."

Cummins was replaced by Tim Griffin, a former aide to Taylor and Rove.

With lawmakers challenging Bush's claim of executive privilege, the battle will likely go to the courts unless the White House and Congress reach a compromise.