BEIRUT — Syrian rebels backed by the United States are making their biggest gains yet south of 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 United States 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 Jordan. The group known as the Friends of Syria, including Jordan, France, the United States 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.

The gains are a contrast to northern Syria, where U.S.-backed rebels are collapsing in the face of an assault by Islamic militants. Notably, in the south, the rebels are working together with fighters from al-Qaida’s Syria branch, whose battle-hardened militants have helped them gain the momentum against government forces. The cooperation points to the difficulty in American efforts to build up “moderate” factions while isolating extremists.

“The goal is to reach the capital … because there is no way to bring down the regime without reaching Damascus,” said Ahmad al-Masalmeh, an opposition activist in Daraa.

But few are under the illusion that the offensive in the south can loosen Assad’s grip on power in the near future. The Syrian leader has benefited from the U.S.-led coalition’s war against the Islamic State, which has had the side effect of freeing up Assad’s forces to focus on more moderate rebels elsewhere in the country. Government forces have seized several key areas around the capital.

Jbawi said the international support for the assault “is not enough to let the rebels win the battle militarily. They are backing (us) to pressure Bashar Assad’s regime to bring him to the negotiating table.”

The Islamic State group’s onslaught in Syria and Iraq has given greater urgency to international efforts to find some sort of solution for Syria’s conflict, which has killed more than 200,000 people and displaced millions. Previous attempts and two rounds of peace talks in Switzerland this year failed to make any progress as each side remained convinced it could win the war militarily.

The U.N. envoy to Syria, Staffan de Mistura, has now proposed local cease-fires starting with the northern city of Aleppo as a building block for a wider solution – an idea that Assad has said is “worth studying.”

Jbawi said 54 rebel factions consisting of 30,000 fighters are taking part in the battles in southern Syria. Activists say that Jordan is also facilitating the rebels’ push by arming rebels and allowing them to cross freely to and from the country.