WASHINGTON — The Pentagon is investigating the cause of a crash of an American military surveillance drone off the coast of Yemen Monday morning, two U.S. officials said.

The officials, speaking on the condition of anonymity to discuss operational matters, confirmed that the drone, an MQ-9 Reaper, fell out of the sky. Iranian-backed Houthi militants said on Monday that they had downed the drone near the port city of Hodeida, in western Yemen.