A close alliance between the US-led coalition battling the Islamic State jihadist group and Syrian Kurdish fighters has racked up successes, but also stirred resentments that could hamper the campaign.
Analysts warn that IS can only be defeated in Syria with support from Sunni Arabs, many of whom are being alienated by what they see as US favouritism towards the Kurdish minority.
In recent months, fighters from the Kurdish People's Protection Units (YPG) have repelled several IS attacks and deprived the group of its key border bastion of Tal Abyad.
US-led air strikes have been key to these advances, and Washington has praised the Kurds as a reliable ally.
But in areas where ties between Kurds and Arabs are already tense, the alliance is viewed with suspicion and anger.
Osama Abu Zeid, a legal adviser to rebel groups under the Free Syrian Army banner, slammed Washington for playing favourites.
"We reject this policy of supporting parties based on ethnic or sectarian affiliation," he said, accusing the US-led coalition of sowing mistrust.
The Kurds "are seen as the darlings of the West," said Emile Hokayem, a senior fellow and Syria expert at the International Institute for Strategic Studies.
"They get intelligence, military support and weaponry, they get the media attention, while the Arabs would say they are suffering more, they are fighting IS and they are fighting (President Bashar al-) Assad."
"The US coalition strategy has favoured the Kurds and, in the process, has alienated important Arab constituencies," Hokayem said.
History of tensions
Suspicion and tensions between Kurds and Arabs in parts of northern Syria have deep roots.
From the 1970s, the Damascus regime resettled Sunni Arabs in traditionally Kurdish areas in a bid to dilute Kurdish nationalist sentiment and win support from Arab tribes.
A rebel fighter from the "First Battalion" under the Free Syrian Army takes part in a military training on June 10, 2015, in the rebel-held countryside of the northern city of Aleppo.
Fierce competition for resources also created tensions between ethnic groups, particularly during the drought years that preceded the Syrian uprising.
More recently, the Syrian opposition has been angered by the Kurds' failure to join the uprising.
Instead, Kurds have maintained a tacit deal with the regime to stay neutral and focused on securing Kurdish-majority areas and building local governance as part of a dream of self-administration.
That policy has strained ties between the main exiled opposition National Coalition and the largest Kurdish group, the YPG's political wing, the Democratic Union Party.
Relations have deteriorated further in recent weeks as the Kurds have clawed back large swathes of territory from IS, sparking a furious war of words.
The exiled opposition has accused YPG fighters of "terrorism" and "widespread violations" against civilians, including looting and the ethnic cleansing of Arabs and Turkmen from captured villages.
Such allegations are angrily rejected by the YPG, particularly after a spate of atrocities against Kurdish civilians by IS.
"Daesh (IS)... is committing the most heinous crimes against humanity. And when we expel them, the Coalition comes out against us with false allegations," it complained.
Human rights groups are still investigating the allegations, although the small, mostly Arab Burkan al-Furat rebel force that fights alongside the Kurds says the claims are "politicised".
Kurdish affairs analyst Mutlu Civiroglu noted that the Coalition issued no similar condemnation when IS fighters killed more than 200 Kurds during a recent incursion into the border town of Kobane.
Limits of US strategy
Charles Lister, a visiting fellow at the Brookings Doha Centre think-tank, said ties between the Sunni Arab opposition and the Kurds were at a "new low."
"Since US-led coalition strikes began... the Kurdish YPG has been the clear benefactor, while Sunni Arab rebels have been placed under tough vetting procedures before even being considered viable partners," he said.
On the ground, Sunni Arab residents of Tal Abyad minced no words about the Kurdish fighters who seized the city.
Seyh Deham Haseki, 60, described IS as a "much lesser evil compared to the Kurdish militants".
"We won't accept the Kurds because this isn't their land. It has always been the Arabs' land. We will stand against them until the very end."
Hokayem said the exclusive coordination between the US-led coalition and the Kurds was playing into a potentially dangerous Sunni Arab narrative in Syria.
They feel "that Arab suffering, and Sunni Arab suffering particularly, is second to that of all other communities," he said.
Eventually, that may undermine the war against IS because Kurdish forces will not fight to capture non-Kurdish areas like the jihadists' de facto Syrian capital Raqa, and Sunni Arab participation will be needed.
"We are reaching the limits of the US strategy," Hokayem said, urging Washington to "define, articulate and implement a serious anti-Assad policy".
"Only when Arab Sunnis feel that their own grievances are recognised, acknowledged and addressed will they be able to see the Kurds as potential partners in the anti-IS fight."