WASHINGTON - Osama bin Laden may be slipping back and forth from Pakistan to Afghanistan. Or the U.S. might not have a clue, more than eight years after the al Qaeda leader masterminded the terrorist attacks on America.

Given a chance yesterday to clear away some of the mystery surrounding the whereabouts of the world's most wanted terrorist, Obama administration officials seemed to add to it with what appeared to be conflicting assessments.

James Jones, President Obama's national security adviser, said that bin Laden, believed hiding mainly in a rugged area of western Pakistan, may be periodically slipping back into Afghanistan. But Obama's Pentagon chief, Robert Gates, said that the United States has lacked good intelligence on bin Laden for a long time - "I think it has been years" - and did not confirm that he'd slipped into Afghanistan.

The failed hunt for bin Laden has been one of the signature frustrations of the global war on terrorism that former President George W. Bush launched after the Sept. 11 attacks. The main explanation given by both the Bush and Obama administrations for not getting bin Laden is that they simply don't know where he is.

"If we did, we'd go get him," Gates said.

Jones, a retired Marine general, stressed the urgency of targeting bin Laden, and spoke of a renewed campaign to capture or kill him. Bin Laden had been sheltered in Afghanistan by Taliban allies while plotting the Sept. 11 attacks. When U.S. forces ousted the Taliban in late 2001, bin Laden fled into Pakistan from his mountain redoubt.