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.

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 militants.

"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 group, 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.

Rebels in the south say they hope the new push will be just enough to pressure Assad to negotiate a peaceful solution to the conflict.

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 earlier this year failed to make any progress as each side remained convinced it can 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."

Speaking by telephone, 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 some rebels and allowing them to cross freely to and from the country.

The rebel offensive gained momentum two months ago, leading to the capture of much of the Quneitra region bordering Syria's Israeli-occupied Golan Heights, as well as large areas in the southern province of Daraa on the border with Jordan.

These included the town of Nawa and the Harra hill, a strategic hill where Syrian troops had stationed monitoring equipment because of its proximity to Israeli army positions in the Golan. The hill, one of the highest in Daraa province, also overlooks a main road that rebels use.

More recently, the fighting has been concentrated in and around the contested village of Sheikh Maskeen and the nearby Brigade 82 base, one of the main government units in the province. If the rebels capture the village and the base they will be then able to threaten the Damascus-Daraa highway, a main lifeline for government forces.

The rebel offensive could eventually link opposition fighters' positions in Daraa and Quneitra with Damascus' rebel-held Ghouta suburbs.

"The military objective is to secure lines of communication and to put pressure on the capital," said Faysal Itani, a resident fellow at the Atlantic Council.

However, despite the rebel advance, Assad's forces remain strong in the area, holding bases in critical locations that the rebels will find difficult to capture, he said.

Daraa-based activist Ibrahim Hariri said that while government forces collapsed in some parts of the province, they still hold much of the city of Daraa and control the Daraa-Damascus highway, "the spine of the province."

"The regime always has a very big force in Daraa because it is close to the front with Israel," Hariri said. "Any attempt to reach Damascus will not be an easy mission."