Pro-government fighters in Yemen have besieged a military base held by Shiite Houthi rebels for 40 hours after cutting off major supply routes to the facility, Yemeni military officials said Tuesday. The officials, speaking on condition of anonymity because they were not authorized to speak with reporters, said the fighters besieged the al-Anad base in Lahj province as part of their push north from the southern port city of Aden, which they captured earlier this month. In the nearby strategic town of Sabr, 20 Houthi fighters were killed in clashes, residents and security and medical officials said. The fighting in Sabr, which lies on a key supply route, has intensified as the rebels have brought in reinforcements from Lahj. Dozens of families fled south to Aden riding on camels and in cars, witnesses and security officials said. At least 25 pro-government fighters were killed Monday in multiple fronts in southern Yemen, said medical and security officials. The fighting in Yemen has raged on despite a unilateral halt late Sunday in the Saudi-led air campaign against the rebels. The officials said troops trained by the Saudi-led coalition are reinforcing pro-government fighters.