Karl Rove has agreed to make a late-stage appearance before a grand jury investigating whether White House officials broke the law by exposing a covert CIA operative, underscoring the increasingly central role of the president's top political adviser in the inquiry.

The intensifying scrutiny comes at a difficult time for the White House. Mr Rove has been busy this week trying to calm conservative criticism of Harriet Miers, President George W. Bush's selection for the Supreme Court, while Mr Bush faces continued public dissatisfaction with his handling of the war in Iraq and his response to Hurricane Katrina.