President Donald Trump withdrew the U.S. from the 2015 Iran nuclear deal two years ago but his administration argues it retains the right to invoke the “snapback” of U.N. sanctions because it is a council member. Others disagree.

Hook did not give a reason for leaving, although he has young children and was frequently away from home on travel. He had just returned to the U.S. from an extended trip to the Middle East and Europe during which he tried to drum up support for the Iran arms embargo extension.