US expresses concern after Iran threatens to shoot down warplanes

US State Department spokesman Mark Toner said Tuesday that the recent confrontations and provocations by Iran against American planes and vessels are "concerning."  He was referring to recent simulated attacks by armed Iranian speedboats on US Navy ships, as well as to Monday's incident in which two US warplanes in international airspace above the Strait of Hormuz were contacted by radio by Iran's air defense network and told to change course or be shot down by ground-to-air missiles. When asked by reporters whether Iran's nuclear deal with Western powers had made Tehran more confrontational, Toner said "I can't rule that out."