U.S. Air Force F-16 fighter jets fly over the Osan U.S. Air Base during a combined air force exercise with the United States and South Korea in Pyeongtaek, South Korea, Dec. 4, 2017. The U.S. could have the first Ukrainian pilots trained on F-16 fighter jets before the end of the year, though it will be longer than that before they are flying combat missions. That's according to the head of the Air National Guard force tasked with training them. (AP Photo/Ahn Young-joon) **FILE**
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