“No matter where the Syrian conflict ends up going, no matter who ends up winning, al Qaeda will stay in Syria for a long period of time,” said Charles Lister, of the global security analysis firm IHS Jane’s Terrorism and Insurgency Center in London.

An estimated 10,000 to 12,000 members of al Qaeda and its affiliates are in Syria, said a U.S. government official who spoke on the condition of anonymity to discuss security issues.

Al Qaeda’s “core” in Pakistan and Afghanistan numbers about 300 members. Its branch in the Arabian Peninsula has about 1,000 followers, and its affiliate in North Africa claims 300 to 1,000 fighters, said Robin Simcox, a terrorism analyst for the British think tank Henry Jackson Society.

Mr. Simcox’s study “Al Qaeda’s Global Footprint: An Assessment of Al Qaeda’s Strength Today” notes that the terrorist network’s second-largest safe haven is Somalia, where al-Shabab has 4,000 to 8,000 members.