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-Qaida leader masterminded the terrorist attacks on America.

Given a chance Sunday 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.

President Barack Obama’s national security adviser, James Jones, said 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 the U.S. 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.

Asked on CNN’s “State of the Union” whether the administration has reliable intelligence on bin Laden’s whereabouts, Jones replied, “The best estimate is that he is somewhere in North Waziristan, sometimes on the Pakistani side of the border, sometimes on the Afghan side of the border.” He did not comment on the intelligence behind that estimate, nor did he cite a time period or describe more specifically bin Laden’s apparent border crossings.

Gates told ABC’s “This Week” that “we don’t know for a fact where Osama bin Laden is,” although he agreed that his likely location is North Waziristan.

That’s part of the loosely governed Federally Administered Tribal Areas of northwest Pakistan where the border with Afghanistan is largely unrecognized and unmarked. There is little Pakistani government or military control in this remote region, and militants affiliated with al-Qaida can move freely across the frontier into Afghanistan.

The U.S. has targeted North Waziristan and other areas on the Pakistan side of the border with drone-launched missile strikes, killing substantial numbers of militants as well as Pakistani civilians. The Pakistani army has undertaken an offensive against Taliban militants in South Waziristan but it has not expanded the effort into North Waziristan.

Obama administration officials have often asserted, as did the Bush administration, that they believe bin Laden is being sheltered on the Pakistani side of the border, along with other senior al-Qaida leaders. But Jones broke new ground by saying publicly that the al-Qaida chief may have slipped back into Afghanistan.

Sen. John McCain, R-Ariz., made a somewhat similar, if less specific, remark Sunday about bin Laden’s movements. He told NBC’s “Meet the Press” that knowledgeable people have told him that bin Laden “moves back and forth.”

McCain did not elaborate, except to say that although bin Laden is not currently able to establish bases for training and equipping terrorists who would attack the United States, “I think it’s important to get him.”

Two Afghan provinces in the country’s northeast held particular attraction for bin Laden in the 1990s: Kunar and Nuristan. The towering mountains there hid bin Laden training camps that date back to the Soviet occupation of Afghanistan in the 1980s. A longtime bin Laden ally, Gulbuddin Hekmatyar, holds sway in the area.