SANAA, Yemen (AP) — A third day of fighting in Yemen's capital pitting Shiite rebels against Sunni militiamen and army units killed at least four civilians and three fighters on Saturday, security and health officials said. Mortar shells slammed into homes and a local hospital as the Shiite rebels, known as Hawthis, battled militias and troops allied with the Muslim Brotherhood's Islah party. The fighting has raised fears of an all-out sectarian conflict in an impoverished country already grappling with a powerful local al-Qaida affiliate and an increasingly assertive separatist movement in the south. The fighting on Saturday took place on the road to the international airport and near a major military base, the state television building and on two university campuses, north and west of the capital. Shells from the fighting around the base hit the walls of Sanaa University nearby, prompting authorities to cancel classes there as well as at public and private schools.