Drones have repeatedly swarmed sensitive military sites. Current and former Air Force generals, along with a senior member of Congress, say the incursions pose a security threat.

Drones have been flying over critical civilian and military sites around the United States for years. Why is it so hard for the military to stop them?

The death toll from US strikes on Yemen has risen to 53, including five children, the Houthi rebels' health ministry said.