DUBAI - An air strike hit the area of the Mazraq refugee camp in the northern Yemen district of Haradh on Monday, killing 21 people, humanitarian workers told Reuters.One humanitarian official said the bombing had targeted a military installation in the vicinity.A Saudi-led coalition is bombing Iran-allied Houthi militiamen and military units fighting alongside them as they advance to the south, towards the port city of Aden, the last bastion of President Abd-Rabbu Mansour Hadi.