Shiite fighters, known as Houthis, wearing army uniforms escort the convoy of Ismail Weld al-Sheikh, new U.N. special envoy to Yemen, at the international airport in Sanaa, Yemen, Tuesday, May 12, 2015. Warplanes from a Saudi-led coalition kept up their airstrikes targeting the positions of Yemen's Shiite rebels and their allies around the capital, Sanaa, hours ahead of a humanitarian cease-fire set to begin on Tuesday evening. (AP Photo/Hani Mohammed)