A U.S. military surveillance drone was shot down off the coast of Yemen on Wednesday by Iran-backed Houthi rebels, the Pentagon said. Pentagon officials, speaking on the condition of anonymity to discuss operational matters, confirmed that the drone, an MQ-9 Reaper, had been shot down. But they would not say if the aircraft was armed, where it was flying from or other details. The downing of a Reaper drone, the mainstay of the American military's aerial surveillance fleet, was the latest escalation of violence between the United States and Iran-backed groups in Yemen, Iraq and Syria. The episodes have underscored the risks that the conflict between Israel and the Palestinian group Hamas could spiral into a wider war.