In a shift with profound implications, the Bush administration is attempting to re-energize its terrorism-fighting war efforts in Afghanistan, the original target of a post-September 11 offensive. America also is refocusing on Pakistan, where a regenerating Al Qaeda is posing fresh threats.

There is growing recognition that America risks further setbacks, if not deepening conflict or even defeat, in Afghanistan, and that success in that country hinges on stopping Pakistan from descending into disorder.

Privately, some senior American military commanders say Pakistan’s tribal areas are at the center of the fight against Islamic extremism; more so than Iraq, or even Afghanistan. These areas border on eastern Afghanistan and provide haven for Al Qaeda and Taliban fighters to regroup, rearm, and reorganize.

At a Pentagon news conference last week, Defense Secretary Gates said that while America respects the Pakistani government’s right to decide what actions are needed to defeat extremists on its soil, there are reasons to worry that Al Qaeda poses more than an internal threat to Pakistan.

“I think we are all concerned about the re-establishment of Al Qaeda safe havens in the border area,” Mr. Gates said. “I think it would be unrealistic to assume that all of the planning that they’re doing is focused strictly on Pakistan. So I think that that is a continuing threat to Europe as well as to us.”

The Pentagon says it has fewer than 100 troops in Pakistan, including personnel who are training Pakistan’s paramilitary Frontier Corps in the western tribal region along the Afghanistan border.

The American military has used other means, including aerial surveillance by drones, to hunt Osama bin Laden and other senior Al Qaeda leaders believed to be hiding near the Afghan border. Ground troops on the Afghan side sometimes fire artillery across the border at known Taliban or Al Qaeda targets, and American officials have said special operations forces are poised to strike across the border under certain circumstances.

In recent days, administration officials have said they would send more American forces, including small numbers of combat troops, if the Pakistani government decided it wanted to collaborate more closely.

It is far from certain that American combat troops will set foot in Pakistan in any substantial numbers. On Friday, President Musharraf of Pakistan said his country opposes any foreign forces on its soil. “The man in the street will not allow this — he will come out and agitate,” he said.

The top two American intelligence officials made a secret visit to Pakistan in early January to seek Mr. Musharraf’s permission for greater involvement of American forces in trying to ferret out Al Qaeda and other militant groups active in the tribal regions, a senior American official said Saturday. Mr. Musharraf was said to have rebuffed an expansion of an American presence in Pakistan at the meeting, either through overt CIA missions or by joint operations with Pakistani security forces.