WASHINGTON — Any future terrorist attack on America probably would originate in Pakistan’s western tribal regions, where Al Qaeda leaders have set up their most secure haven since the fall of the Taliban in Afghanistan, the top American military officer said yesterday.

But Admiral Michael Mullen, chairman of the Joint Chiefs of Staff, said forcible action to disrupt the terrorists’ planning effort is unlikely for now. Difficulties faced by the new government in Islamabad, including a deteriorating economy, are forcing American and Pakistani leaders to wait before moving against extremists.

“I believe fundamentally if the United States is going to get hit, it’s going to come out of the planning that the leadership in the FATA is generating, their planning and direction,” Admiral Mullen said, referring to Pakistan’s Federally Administered Tribal Areas. “I’m not saying it is guaranteed it’s going to happen or that it’s imminent. But clearly we know the planning is taking place.”

Admiral Mullen’s comments echoed the thinking among many military and intelligence officials but represent one of the starkest assessments yet about the implications of Al Qaeda’s ability to reorganize in western Pakistan. Admiral Mullen visited the country’s troubled border region last week.

The Pashtun tribal regions have become the primary focus of worldwide American counterterrorism efforts amid intelligence reports that Al Qaeda has been able to regroup to levels not seen since the American invasion of Afghanistan seven years ago.

Admiral Mullen’s visit, which included meetings with Pakistan’s new Army chief, Ashfaq Kiyani, was his third in just six months but his first since the new Pakistani government took office in late March.