For the United States, the movement creates opportunities as well as risks. With the Obama administration focusing its fight against the Taliban and Al Qaeda on the havens in Afghanistan and Pakistan, a shift of fighters and some leaders to new locations could complicate American efforts to strike a lasting blow.

But in the tribal areas of Pakistan, Qaeda and Taliban forces have drawn for protection on Pashtun tribes with whom they have deep familial and tribal ties. A move away from those areas could expose Qaeda leaders to betrayal, while communications among militants in Pakistan, Somalia and Yemen have created a new opportunity for American intelligence to zero in on insurgents who gave up many electronic communication devices shortly after the Sept. 11 attacks to avoid detection.

A senior Obama administration official attributed some of the movement to “the enormous heat we’ve been putting on the leadership and the mid-ranks” with Predator strikes, launched from both Pakistan and Afghanistan. Mr. Obama’s strategy so far has been to intensify many of the strikes begun under the Bush administration.

“There are indications that some Al Qaeda terrorists are starting to see the tribal areas of Pakistan as a tough place to be,” said an American counterterrorism official. “It is likely that a small number have left the region as a result. Among these individuals, some have probably ended up in Somalia and Yemen, among other places. The Al Qaeda terrorists who are leaving the tribal areas of Pakistan are predominantly foot soldiers.”

Measuring the numbers of these movements is almost as difficult as assessing the motivations of those who are on their way out of the tribal areas.

But American officials say there is evidence of a shift. One senior American military official who follows Africa closely said that more than 100 foreign fighters had trained in terrorism camps in Somalia alone in the past few years. Another senior military officer said that Qaeda operatives and confederates in Pakistan, Yemen and Somalia had stepped up communications with one another.

“What really has us worried is that they’re communicating with each other much more  Al Qaeda in Pakistan, Somalia and Yemen,” the senior military officer said. “They’re asking, ‘What do you need? Financing? Fighters?’ ”