The U.S.-made RQ-4 Global Hawk spy plane looks like an upside-down double-decker bus with wings, flies slowly, offers zero leg room — and is one of the most coveted pieces of military technology in the world.

Impressed by its successes in combat for the U.S. Air Force in Iraq and Afghanistan, countries around the globe are lining up to buy the unmanned drone aircraft to bolster their own intelligence abilities, despite concerns that exports might send sensitive technology into the wrong hands.

Some experts said the export of the planes could also heighten tensions with China, Iran and Russia, which could be the subject of closer observation and perceive the drones' operations as offensive threats.