Reuters reports al Qaeda-affiliated Sunni Muslim insurgents killed 18 Shi'ite Muslim Houthi rebels and seized an important city in central Yemen, tribal sources said on Monday, in a spread of sectarian conflict as central government authority has unraveled. The northern-based Houthis established themselves as power brokers in Yemen last month by capturing the capital, Sanaa against scant resistance from the weak administration of President Abd-Rabbu Mansour Hadi. Houthi forces have since advanced into central Yemen and taken on Sunni tribesmen and al Qaeda militants, who regard the Houthis as heretics. Fighting has flared in several provinces, alarming the world's No. 1 oil exporter Saudi Arabia next door.