WASHINGTON -- 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-Sept. 11 offensive. The U.S. also is refocusing on Pakistan, where a regenerating al-Qaida is posing fresh threats.

There is growing recognition that the United States 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 U.S. 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-Qaida and Taliban fighters to regroup, rearm and reorganize.

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 U.S. military has used other means, including aerial surveillance by drones, to hunt Osama bin Laden and other senior al-Qaida 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-Qaida targets, and U.S. 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 U.S. forces, including small numbers of combat troops, if the Pakistani government decided it wanted to collaborate more closely.

It is far from certain that U.S. combat troops will set foot in Pakistan in any substantial numbers. On Friday, Pakistan's president, Pervez Musharraf, 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. Musharraf said the U.S. instead should boost its combat forces in Afghanistan.

The number of U.S. troops in Afghanistan has grown over the past two years from about 20,000 to 28,000. That is the highest number of the war, which began in October 2001. The total will increase by 3,200 this spring with a new influx of Marine reinforcements, including 2,200 combat troops who will bolster a NATO-led counterinsurgency force in the south.

"There is strong pressure now from the international community to find some solution to Afghanistan because of the fear that this could quickly go south," said Ashley J. Tellis, a senior associate at the Carnegie Endowment for International Peace.

"We haven't lost the war yet, but we could be on our way to doing so," Tellis said in a telephone interview Friday. He strongly recommends strengthening the U.S. military presence in southern Afghanistan.