BEIRUT -- Drawing on thousands of combatants from Syria's mix of religious and ethnic groups, a U.S.-backed alliance called the Syrian Democratic Forces has emerged as the most effective fighting force against the Islamic State group.
But the dominant role of Kurdish fighters in the alliance is a concern for majority Sunni Arab factions and their regional backers, raising questions about the group's future role in a broader political context in Syria.
The coalition, which focuses on fighting the Islamic State, already faces opposition from other groups fighting to topple President Bashar Assad because those groups widely distrust the Kurds.
And while the coalition has been capturing territory steadily in northern Syria from the extremists, it is hampered greatly by its inability to retake areas with a majority Sunni Arab population.
In a devastated landscape where extremists and Islamic groups largely preside, the Syrian Democratic Forces are the closest thing to an inclusive and moderate fighting force in Syria. It represents the largest of the nongovernment fighting forces arrayed against IS in Syria, with some estimates putting the number of fighters affiliated with the group at nearly 40,000.
"The Syrian Democratic Forces are the most organized in the Syrian chaos," said Kurdish activist Mustafa Bali, speaking from the Kurdish town of Kobani in northern Syria. He said the group has a united command stretching from the predominantly Kurdish town of al-Malikiyah in the east to Afrin in the west, with new members joining the alliance on daily basis.
The group is led by the main Kurdish fighting force in Syria, the People's Protection Units, known as the YPG. It seeks to build on the success of the Euphrates Volcano, an alliance of Kurd and Arab factions that last year liberated Kobani from Islamic State militants.
It includes Arab forces such as the Sanadid force, mainly drawn from the Arab Shammar tribe; a Christian militia known as the Syriac Military Council, which includes Assyrians; the mainly Arab Jazira Brigades; the Seljuk Brigade, which consists mostly of Turkmen forces; and the Jaysh al-Thuwar group, which includes U.S.-backed rebels who were routed from Idlib and Aleppo provinces earlier this year by the al-Qaida branch in Syria, the Nusra Front.
Most of those groups are small and poorly equipped. Apart from the YPG, they lack military capabilities beyond defending their towns and villages.
The alliance was announced in the predominantly Kurdish province of Hassakeh on Oct. 10, a day after the U.S. said it was abandoning its effort to build a new rebel force inside Syria to combat the Islamic State group, acknowledging the failure of its $500 million campaign to train thousands of fighters.
Instead, the U.S. said it would provide support for those already fighting the IS group, which also is known as ISIS, ISIL and its Arabic acronym, Daesh.
A few days after the new alliance was formed, the U.S. dropped weapons and ammunition to Arab forces in the alliance -- an apparent effort to allay Turkish concerns about Washington's support for the YPG. The YPG is seen by Turkey as an extension of the Kurdish PKK, which has waged a long insurgency against Ankara.
The alliance has said it wants to liberate Hassakeh province from IS and move on to the militants' stronghold in Raqqa. U.S. officials have announced plans to deploy dozens of special operations forces in Syria to support it.
Last month, it successfully dislodged IS from the strategic town of Hol near the border with Iraq. That came just as the Kurdish peshmerga in Iraq took over the town of Sinjar from the extremists, cutting off a main route from Raqqa to IS holdings in Iraq and making movements of fighters and supplies more difficult. It also has captured towns and villages in the province and is moving south toward the IS stronghold of Shaddadeh.
Col. Talal Sillu, a spokesman for the Syrian Democratic Forces, said although they have not received weapons from the U.S. beyond the one airdrop, he is satisfied with the U.S.-led coalition's response to their airstrike requests.
"They are participating with us effectively, and they are helping us in the victory against the Daesh terrorist organization," he told The Associated Press in a telephone interview.
In its founding statement, the Syrian Democratic Forces said its aim beyond destroying IS was to build a democratic, pluralistic Syria "where all Syrian citizens of all sects enjoy freedom, justice and dignity."
"Officially they represent a whole range of ethnicities and ostensibly the vision could be deemed moderate, but the coalition can only gain limited traction, as the YPG is justifiably perceived as the dominant actor to which the allied rebel groups in particular are junior partners," said Aymen Tamimi, an expert on rebel and Islamic extremist groups and a fellow at the Middle East Forum think tank.
One of the alliance's biggest challenges is reclaiming mostly Arab areas with a fighting force whose most effective combatants are Kurds.
"They added Arab groups to the alliance to dilute the Kurdish element, but everyone knows it's the Wihdat," said Abu Khaled, a rebel fighter loosely affiliated with the Free Syrian Army, using the Arabic abbreviation for the YPG.
"Their (YPG) record is not clean, and that is the biggest problem facing this alliance," he said from Turkey, where he goes back and forth to Syria.
His comments reflected the wide distrust the YPG faces among mainstream rebels in Syria.
Many Arabs in northeastern Syria are wary of the Kurds' creation of semiautonomous areas and have accused the YPG of ethnic cleansing and mass demolition of homes in Sunni areas captured from IS, including the key town of Tal Abyad in the summer. Amnesty International accused the group of committing war crimes in Tal Abyad.
Charles Lister, a visiting fellow at the Brookings Doha Center, said the blending of Arab tribes, Kurds and Christians with allied FSA factions is a "hugely beneficial development" -- in theory.
"But the YPG's overwhelmingly dominant role within the SDF has given the coalition a certain tinge that the vast majority of the conventional opposition are strongly in opposition to," he said.
The distrust has been reinforced by clashes this week between mainstream rebels from the FSA and fighters from Jaish al-Thuwar, which is part of the Syrian Democratic Forces. Most of the fighting has been in a border area where Turkey is examining the possibility of creating a safe zone for civilians and Turkish-backed rebels fighting Assad's forces.
A U.S. official acknowledged the group's success in cutting IS supply lines in remote areas of eastern Syria but suggested the real test would be trying to retake a Sunni city.
"Their initial foray was good. They took some territory, they cut supply lines -- all of that is very encouraging," the official said, speaking on condition of anonymity because he was not authorized to talk to the media.
With a political process steadily progressing, it is unclear what role the alliance can claim in Syria's future.
Sillu, the SDF spokesman, said alliance representatives have not been invited to an upcoming meeting of Syrian opposition figures in Saudi Arabia, where a delegation will be picked to send to talks with representatives from Assad's government.
That could lead to "infighting between us and other factions, since there are groups that will go and negotiate on our behalf," he said.
Sillu, an ethnic Turkmen who was in the Syrian army for 21 years before retiring in 2005, dismissed suggestions the Kurds are the backbone of the force, saying they simply have the most experience fighting IS.
"Our project is a national one, but we don't deny that at the beginning those who gave sacrifices are our Kurdish brothers. Then others joined," he said.
Associated Press writer Ken Dilanian in Washington contributed to this report.
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