By Tom Perry and Suleiman Al-Khalidi
BEIRUT/AMMAN (Reuters) - Rebels in southern Syria say they have taken a step toward unity that may attract more support from their Western and Arab backers, forging a joint defense pact to help shield them from government forces and Islamic State.
The south is the last major stronghold of the mainstream opposition to President Bashar al-Assad following the expansion of Islamic State in the east and north and gains by the al Qaeda-affiliated Nusra Front in the northwest.
A short drive from Damascus, the southern front remains a risk for Assad as he shores up his control over key areas of central Syria. Insurgent groups including both mainstream rebels and Nusra have made slow but steady gains in the south against government forces, analysts who track the conflict say.
As the United States seeks partners on the ground for the campaign against Islamic State, the southern rebels are trying to address long-standing criticisms of the so-called moderate opposition by getting better organized.
"We are moving in steps. The joint defense agreement is part of the complete plan for uniting the southern front," said Bashar al-Zoubi, head of the Yarmouk Army -- one of the biggest rebel groups in the south. He spoke to Reuters via the internet.
The agreement dated Dec. 6 and signed by 17 rebel leaders was seen by Reuters. It follows an agreement among the southern groups on a transition plan for Syria.
With the war about to enter its fourth year, analysts say rivalries among a plethora non-jihadist groups have been one of their major weaknesses. The Turkey-based Syrian political opposition has little or no sway over the armed groups.
Rebels in the north last week unveiled a separate initiative grouping mainstream factions with hardline Islamists including Ahrar al-Sham -- a group U.S. Secretary of State John Kerry has equated with Islamic State. That makes partnering with it a complicated prospect for Assad's Western opponents.
The separate initiatives underscore the divergent paths of the war. The southern rebels believe resolving the conflict in the south is easier than resolving it in the north, where the jihadists are dominant.
They advocate an approach that would see the war in the south resolved first. Their transition plan safeguards state bodies and guarantees the rights of religious minorities who worry the alternative to Assad is al Qaeda.
Rebels in the south have already received what they describe as small amounts of military and financial support from Western and Arab states. It has been channeled via Jordan, a U.S. ally that is determined to protect its Syrian frontier from jihadists.
The main Arab backers of the southern groups are Saudi Arabia and the United Arab Emirates. Qatar, which has funneled aid to Islamists in the north, has been kept out.
President Barack Obama praised Jordan's support for the "moderate" Syrian opposition during a Dec. 5 meeting with King Abdullah, an apparent reference to the southern rebels.
Though they have received support including American-made anti-tank missiles, it is not yet clear how these groups may fit into U.S. plans for building an opposition force to confront Islamic State, which is more of a threat in the north for now.
Abu Hamza al-Qabouni, head of a Southern Front group from Damascus, said the rebels aimed to coalesce into something resembling an army. "The main gain of this agreement is that it makes unification easier," he said.
"If we're talking about the type of threat, we say naturally it is the regime, with Daesh as the second target," he added, using a derogatory Arabic acronym for Islamic State.
The south is currently scene of one of the fiercest battles between the government and its armed opponents in and around the town of Sheikh Maskin. State media reports have said the army has been inflicting daily losses on the insurgents there, all of whom it describes as terrorists.
Jordanian military analyst retired general Mamoun Abu Nowar said the rebels were gaining incrementally. "They are moving in the direction of a centralized command," he said. "There is an external foreign dimension that is allowing them to work this way," he added, pointing to the foreign backing.
Nusra is fighting alongside the Western-backed groups in the south, though the rebels say they do not coordinate with it. The rebels say their concern about Nusra has grown since it routed two mainstream opposition groups in the northwest in October.
Mohammad Mahameed, the head of another Southern Front group, forecast further advances. "After we implement this pact there will be even more progress on the ground," he told Reuters.
"Our backers have a perceptive vision and they see who is effective on the ground and support them."
(Writing by Tom Perry; Editing by Giles Elgood)