DUBAI, March 30 (Reuters) - An air strike hit the area ofthe Mazraq refugee camp in the **rthern Yemen district of Haradhon Monday, killing 21 people, humanitarian workers told Reuters. One humanitarian official said the bombing had targeted amilitary installation in the vicinity. A Saudi-led coalition is bombing Iran-allied Houthimilitiamen and military units fighting alongside them as theyadvance to the south, towards the port city of Aden, the lastbastion of President Abd-Rabbu Mansour Hadi. (Reporting by Mohammed Ghobari and **ah Browning; Editing byLarry King)