Groups that have received support from the United States or its allies have turned their guns on each other in a northern corner of Syria, highlighting the difficulties of mobilizing forces on the ground against Islamic State.
As they fought among themselves before reaching a tenuous ceasefire on Thursday, Islamic State meanwhile edged closer to the town of Azaz that was the focal point of the clashes near the border with Turkey.
Combatants on one side are part of a new U.S.-backed alliance that includes a powerful Kurdish militia, and to which Washington recently sent military aid to fight Islamic State.
Their opponents in the flare-up include rebels who are widely seen as backed by Turkey and who have also received support in a U.S.-backed aid program.
Despite the ceasefire, reached after at least a week of fighting in which neither side appeared to have made big gains, trust remains low: each side blamed the other for the start of fighting and said it expected to be attacked again. A monitoring group reported there had still been some firing.
The fighting is likely to increase concern in Turkey about growing Kurdish sway near its border.
It also poses a new challenge for the U.S.-led coalition which, after more than a year of bombing Islamic State in Syria, is trying to draw on Syrian groups to fight on the ground but finding many have little more in common than a mutual enemy.
Azaz controls access to the city of Aleppo from the nearby border with Turkey. It also lies in an area coveted by Islamic State, which advanced to within 10 km (six miles) of the town on Tuesday and took another nearby village later in the week.
The fighting pitched factions of the Free Syrian Army, supported by Turkey and known collectively as the Levant Front, against the YPG and Jaysh al-Thuwwar - both part of the Democratic Forces of Syria alliance backed by Washington.
The Syrian Observatory for Human Rights, a Britain-based group that monitors the conflict in Syria, said Levant Front was supported in the fighting by the Ahrar al-Sham Islamist group and the al Qaeda-linked Nusra Front.
Observatory director Rami Abdulrahman said the rebels had received "new support, which is coming in continuously" from Turkey, a U.S. ally in the fight against Islamic State.
"Turkish groups against U.S. groups -- it's odd," he said.
Although the YPG has been the most effective partner on the ground for the United States in the fight against Islamic State, Turkey does not want to see its influence expand further, even if the group is fighting Islamic State.
The United States and Turkey have for months been talking of a joint effort to clear Islamic State from the remaining part of the frontier, but there has been no sign of progress.
Long-standing suspicion
The clashes were in villages between predominantly Arab Azaz and the majority-Kurdish town of Afrin further southwest. A few dozen people were killed, including 13 civilians, and small tracts of territory have changed hands.
The insurgents blamed the Kurdish forces and their allies for trying to advance. But a Democratic Forces of Syria spokesman said Islamist groups had attacked first under the pretext that his group was a front for the Kurds.
In Aleppo city, insurgents shelled a Kurdish-inhabited area, and the YPG fired from there at the rebel supply route that leads from Aleppo to Azaz and Turkey, the Syrian Observatory for Human Rights said.
The rivalry is stoked by long-standing rebel suspicion of the Kurdish agenda.
"The Levant Front and the others are in dispute over who should control the Azaz area, and so you have this fighting between them, and the Jaysh al-Thuwwar and the (YPG)," said a rebel leader familiar with the situation. "This is strife between Jaysh al-Thuwwar and the Kurds, with the FSA factions."
Levant Front commander Abu Ahmad al Jazrawi said he hoped Thursday's ceasefire would hold. But if the YPG and their allies "try another time to raid our areas we will not hesitate to attack them again," he warned.
The spokesman for the Democratic Forces of Syria said most of the fighting on its side was being done by non-Kurdish forces, though YPG fighters were reinforcing them from nearby Afrin.
A Levant Front fighter told Reuters the fighting began when the YPG and Jaysh al-Thuwwar seized three villages in the Azaz area. "They cut our main supply route," he said. "We then succeeded in evicting these forces."
Rebels say the YPG has been emboldened by Russian air strikes in Syria that have mostly hit groups fighting President Bashar al-Assad, and are not targeting the Kurds, whose fight with Islamic State has been praised by President Vladimir Putin.
The Syrian Kurds, who have established their own government in areas of northern Syria they control, have been accused by President Bashar al-Assad's opponents of cooperating with him during the 4-1/2-year-old conflict - a charge they deny.
The spokesman for the Democratic Forces of Syria blamed the flare-up on Ahrar al-Sham and the Nusra Front, which he said had initiated hostilities against Jaysh al-Thuwwar positions on the pretext that it was a front for the Kurds.
"Ahrar al-Sham and the Levant Front were dragged behind the lie of the Nusra Front," said Talal Ali Selo, the spokesman, an ethnic Turkmen who is a member of Jaysh al-Thuwwar. The YPG were involved in the fighting on their side, he said.
Selo said he did not trust those groups to keep the peace.
"In my personal opinion, they will not remain committed" to the ceasefire, he said.
The Observatory reported even after it was signed that there was firing in the area, and one rebel group that had been involved in fighting said it was not party to the truce.
New US strategy
The Democratic Forces of Syria alliance, unveiled on Oct. 12, comprises the YPG, Arabs and groups representing other ethnicities. The United States promptly air dropped ammunition to members of the alliance to press the war against Islamic State in northern and eastern Syria, an overhaul of U.S. policy after it abandoned a program to train and equip rebels to fight IS.
The YPG is the strongest element of the coalition, having cleared IS from swathes of eastern Syria with the help of U.S.-led air strikes earlier this year.
The Syrian Kurds already control an uninterrupted 400 km (250 mile) stretch of the Syrian-Turkish border. Turkey's fear is that they aim to link that territory with Afrin by seizing the territory north of Aleppo. Ankara is fighting an insurgency against Kurdish PKK fighters in its southeast.
The border territory on the Syrian side is currently controlled by a mix of rebels and Islamic State, and was the area where Turkey and the United States had been working on plans to crush IS.
U.S. Secretary of State John Kerry said on Nov. 17 the United States was starting an operation with Turkey to finish securing the northern Syrian border - an apparent reference to the area north of Aleppo.
In Aleppo, insurgents shelled the YPG-controlled Sheikh Maqsoud neighborhood, and the YPG opened fire from there on the main road that runs north of the city towards Azaz, the Observatory said.
Abdulrahman said there was no definitive death toll for all the clashes, but that at least 15 rebel fighters and eight on the Kurdish side had died a few days ago, as well as several civilians in Sheikh Maqsoud.
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