WASHINGTON (AP) — The U.S. has pummeled terrorist leaders in Yemen with targeted drone strikes, but the Yemen-based group known as al-Qaida (al-KAH'-ee-duh) in the Arabian Peninsula has proven to be a tenacious enemy.

The group is at the heart of what American officials say is a credible and specific terrorist threat that shut down U.S. diplomatic facilities in 19 cities across the Mideast and Africa.

Officials say al-Qaida leader Ayman al-Zawahri (AY'-muhn ahl-ZWAH'-ree) has reached out to the Yemeni branch to improve cooperation. That could mean the Yemeni terror group wants to target Western interests after a long period of more local and regional focus.

The latest warnings raise questions about how successful America's war on terror has been and whether al-Qaida has reconstituted itself since the death of Osama bin Laden.