Some aides to President Obama attribute the moves to pressure from intensified drone attacks against Qaeda operatives in Pakistan, after years of unsuccessful American efforts to dislodge the terrorist group from their haven there.

But there are other possible explanations. Chief among them is the growth of the jihadist campaigns in both Somalia and Yemen.

Somalia is now a failed state that bears some resemblance to Afghanistan before the September 11, 2001, attacks, while Yemen’s weak Government is ineffectually trying to combat the militants, US officials say.

The shift of fighters is still small, perhaps a few dozen, and there is no evidence that the top leaders — Osama bin Laden and Ayman al-Zawahiri — are considering a move from their refuge in the Pakistani tribal areas.

Most officials would not comment on the record about the details of what they are seeing, because of the sensitivity of the intelligence information they are gathering.

Leon E Panetta, the CIA director, said in remarks here on Thursday that the US must prevent al-Qaeda from creating a new sanctuary in Yemen or Somalia.

The steady trickle of fighters from Pakistan could worsen the chaos in Somalia, where an Islamic militant group, the Shabab, has attracted hundreds of foreign jihadists in its quest to topple the weak moderate Islamist Government in Mogadishu.

“I am very worried about growing safe havens in both Somalia and Yemen, specifically because we have seen Al Qaeda leadership, some leaders, start to flow to Yemen,” Adm Mike Mullen, the chairman of the Joint Chiefs of Staff, said in remarks at the Brookings Institution here on May 18.