Dozens of seasoned militant fighters, including some midlevel planners, have traveled to Syria from Pakistan in recent months in what American intelligence and counterterrorism officials fear is an effort to lay the foundation for future strikes against Europe and the United States.

“We are concerned about the use of Syrian territory by the Al Qaeda organization to recruit individuals and develop the capability to be able not just to carry out attacks inside of Syria, but also to use Syria as a launching pad,” John O. Brennan, the C.I.A. director, told a House panel recently.

The extremists who concern Mr. Brennan are part of a group of Qaeda operatives in Pakistan that has been severely depleted in recent years by a decade of American drone strikes. But the fighters still bring a wide range of skills to the battlefield, such as bomb-building, small-arms tactics, logistics, religious indoctrination and planning, though they are not believed to have experience in launching attacks in the West.

Syria is an appealing base for these operatives because it offers them the relative sanctuary of extremist-held havens — away from drone strikes in Afghanistan and Pakistan — as well as ready access to about 1,200 American and European Muslims who have gone there to fight and could be potential recruits to carry out attacks when they return home.