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 advances appear to be a rare visible success story from efforts by the U.S. and its allies to train and arm moderate rebel fighters.

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. The group known as the Friends of Syria, including Jordan, France the U.S. and Saudi Arabia, are backing the rebels with money and weapons, said Gen. Ibrahim Jbawi, the spokesman for the Free Syrian Army’s southern front. […]