Militiamen in southern Yemen said on Tuesday they had pushed back Iranian-allied Houthi rebels on several fronts, three weeks into a Saudi-led air strikes against the northern movement of Shi'ite Muslim fighters. In the southern port city of Aden, which has seen nearly three weeks of fierce street fighting, Houthi fighters withdrew from the Khor Maksar district, home to an international airport and several foreign missions. The pullout deprives the Houthis of a bridge to downtown districts where they are facing heavy resistance from local fighters. Saudi Arabia and Sunni Arab allies have been bombing the Houthis to try and drive them away from Aden, the last major city in Yemen loyal to President Abd-Rabbu Mansour Hadi, who has fled to Riyadh. The conflict in Yemen is widely regarded as a sectarian proxy war between predominantly Sunni Saudi Arabia and Shi'ite Iran. Riyadh says it is protecting Hadi and his government in exile from the Houthis, but as the world' largest oil exporter, Saudi Arabia is also unhappy at the prospect of protracted upheaval in its southern neighbor. Hadi and his Saudi backers also face fierce opposition from soldiers loyal to former Yemeni president Ali Abdullah Saleh, a shrewd political operator who has teamed up with his old foes the Houthis. Al Qaeda, which has staged suicide bombings against the Houthis, also poses a threat to Yemen's stability. Nearby shipping lanes and the narrow Bab el-Mandeb passage, through which nearly 4 million barrels of oil are shipped daily to Europe, the United States and Asia, could also be at risk from the fighting.