BEIRUT — Syrian rebels backed by the United States are making their biggest gains yet south of the capital Damascus, capturing a string of towns from government forces and aiming to carve out a swath of territory leading to the doorstep of President Bashar Assad’s seat of power. The rebel forces are believed to include fighters who graduated from a nearly 2-year-old CIA training program based in Syria’s southern neighbor Jordan. Notably, in the south, the rebels are working together with fighters from al Qaeda’s Syria branch, whose battle-hardened militants have helped them gain the momentum against government forces. The Syrian leader has benefited from the U.S.-led coalition’s war against the Islamic State group, which has had the side effect of freeing up Assad’s forces to focus on more moderate rebels elsewhere in the country.