A prominent group inside ISIS' de-facto capital had an ominous warning for US plans to defeat ISIS in Syria

Raqqa is Being Slaughtered Silently, the anti-ISIS citizen-journalist organization, said last week that the US' strategy to drive the Islamic State out of its de-facto capital using a Kurdish-dominated brigade risks pushing "a lot of people" inside Raqqa to join the Sunni militant group.

The US-backed, Kurdish-dominated Syrian Democratic Forces (SDF) have been taking back villages within Syria's Raqqa province as it pushes toward the city of Raqqa, which fell to ISIS in mid-2014.

The SDF's Kurdish fighters have said they don't plan on entering the city itself. But Washington's support for the SDF - which is led by the Kurdish People's Protection Units (YPG) - has still unnerved some Syrian Arabs who are suspicious of the Kurds' territorial ambitions and wary of their ties to Russia.

"The Arab fighters are just camouflage," General Salim Idris, the former FSA chief of staff, told Voice of America earlier this month. "The SDF is the YPG, which collaborates with anyone - Assad, the Russians, the Americans - when it suits its purposes."

He added: "I really don't think the Obama administration has thought this through. Will the Kurds give up Arab towns they capture?" he said.

Raqqa is Being Slaughtered Silently (RSS), a citizen-journalist organization founded in 2014 to expose ISIS' brutality and human-rights abuses, went even further by claiming that a Kurdish-led offensive on Sunni-dominated Raqqa would push citizens there to join ISIS, a Sunni extremist group.

"The fact that a US-backed group is criticising the role of the Pentagon-backed YPG against a group that killed some of its colleagues is remarkable," Syrian journalist Hassan Hassan, co-author of "ISIS: Inside the Army of Terror," wrote in The Financial Times. Several members of RSS have been executed by ISIS militants inside Syria and Turkey since 2014.

Hassan noted that divisions between Syria's Sunni Arabs and Kurds "offer ISIS an opportunity to present itself as a custodian" of Sunnis in the cities the group has conquered. The US faces a similar dilemma in Iraq, where ISIS has positioned itself - however disingenuously - as a Sunni alternative to Shiite rule.

Syrian Kurdish and Arab fighters have a long history of mutual distrust that peaked between 2012 and 2013, when the Kurdish People's Protection Units (YPG) battled FSA-aligned rebel groups for control over the Syrian city of Ras al-Ayn.

Those tensions have reemerged over the past eight months. The YPG-controlled neighborhood of Sheikh Maqsood has come under siege by both Syrian government forces and the rebels, with reports emerging that the rebels have committed war crimes against the neighborhood's Kurds, and rebels further north alleging that the Kurds have terrorized Arab civilians and forced them out of their homes.

The rivalry has put the US in a difficult position. The YPG has proven to be the most effective force fighting ISIS on the ground in northern Syria, but the territorial expansion their victories have afforded them has fostered resentment among the country's Arab population - and, in northwestern Syria, has left the rebels caught between the SDF and ISIS.

Americans "just don't trust the Arabs," a Syrian Arab figure told The Financial Times last week. "They say, 'we will look into it, we will do our best.' A lot of 'we wills.' But their actions show that the core they are creating will always be Kurdish."

Washington has struggled to recruit enough Arab soldiers willing to fight alongside the Kurds to liberate cities from ISIS.

Most are either too distrustful of the Kurds to work with them or want to focus on battling forces loyal to Syrian President Bashar Assad. Photos that emerged last week of US troops wearing YPG patches while fighting ISIS in northern Syria - and a visit by the US' top military commander to Kurdish troops earlier this month - have not helped, either.

"People here understand that America wants the area to fall to Daesh, so it can then destroy Azaz on top of Daesh and these terrorist PKK can advance and take the area," Yasser Abu Omar, a cleric based in the Syrian border city of Azaz, told Vice News last week. The PKK is a Kurdish militant group with ties to the YPG.

Other groups, meanwhile, "would probably like to rise up against the Islamic State, but they don't see that as a viable option," Syria expert Aron Lund, a fellow at the Carnegie Endowment for International Peace, told the Military Times in January. "If the US is able to bring a force to the region of Arabs and Kurds and others, with an air of inevitability, then maybe that calculus would change."

Hassan, for his part, was not as optimistic about the US' ability to foment unity within a conflict he and others have characterized as "nakedly sectarian."

"With this campaign, backed by groups abhorred by residents of the two cities" Raqqa and Mosul, the US "seems to have privileged a tactical victory against ISIS over a strategic one," Hassan wrote.

He added: "It looks likely to provide ISIS with a gift it has long wanted, especially in Syria: creating for ordinary people the perception that their choice is between its own jihadis and militias they see as invaders."