A U.S.-backed campaign to take the Syrian city of Raqqa from the Islamic State group is underway under the banner of the predominantly Kurdish Syria Democratic Forces (SDF).

The coalition enjoys the backing of several Western militaries and is recognized by the U.S. as the most effective force fighting against the extremists, but its differences with Turkey as well as other Syrian groups could complicate the campaign.

Here's a look at the SDF:

WHO ARE THE SDF?

The SDF are the general military force of the autonomous Kurdish zone in northern Syria known as Rojava, formed out of a coalition of Kurdish and Arab militias fighting against the Islamic State group in 2015.

It is dominated by the Kurdish Party of Democratic Unity, or PYD, which maintains a tacit non-aggression pact with President Bashar Assad in Damascus. Turkey's government views it as an extension of its country's own outlawed Kurdish insurgency.

A GROWING FOOTPRINT

The PYD has benefited from its quiet agreement with Assad since the early months of the war, when the Syrian army withdrew from much of northeastern Syria to battle rebels elsewhere in the country, allowing the Kurdish party to carve out a zone of autonomy.

The PYD's armed wing, the People's Protection Units, or YPG, later expanded that enclave by battling the Islamic State group with the aid of U.S. airstrikes. The YPG, then the umbrella SDF group, have been driving westward over the past year and a half, and earlier this month captured the town of Manbij, a key IS supply hub west of the Euphrates River.

The SDF coalition now controls all but 50 kilometers (30 miles) of Syria's northern frontier with Turkey.

Victory on the battlefield has brought growing Western support. The U.S. says its 300 special forces servicemen, embedded with the SDF, are performing an advisory role.

TURKISH HOSTILITY

Turkish forces and thousands of allied Syrian opposition fighters launched their own offensive in northern Syria earlier this year, clashing with both the SDF and IS. Turkish President Recep Tayyip Erdogan has suggested his own forces and allies should help take both Raqqa and Mosul, and that Kurdish forces should be sidelined. On Monday Erdogan said the U.S. dependence on the Kurdish forces was "naive."

The U.S. wants all of its allies to set aside their grievances, at least temporarily, and focus on IS.

TOO COZY WITH ASSAD?

The SDF have a mixed reputation among Syrian opposition groups, as well. Hossam Issa of the activist media collective Raqqa is Being Slaughtered Silently says his group's problem with the SDF is that it's coordinating with the Syrian government as a way to further the PYD's Kurdish state building project.

Syrian opposition figures point to SDF advances into opposition-held territory in northern Syria last spring in conjunction with Syrian and Russian airstrikes. The SDF and PYD denied allegations these moves were coordinated.

Critics in the opposition also fear that the PYD and SDF ultimately seek to carve out an independent enclave from northern Syria — a concern shared by Ankara. Riding a wave of military successes, the PYD declared in March that it would insist on a federated solution to the Syria war, alarming most of the conflict's other players who view it as plan for partition.

The PYD shares control of the northeastern city of Qamishli with the government in Damascus. The two authorities also shared control of Hasakah, until the SDF and other PYD-affiliated forces expelled pro-government forces from the city in August.

ALLEGATIONS OF HUMAN RIGHTS ABUSES

Before the SDF was formed, the military-wing of the PYD was accused of human rights abuses in northern Syria. Human Rights Watch in 2014 said the party's military was conscripting children into its forces. Amnesty International in 2015 documented allegations of home demolitions and forced displacement of Arab residents.

The reports have fueled accusations that the PYD, and by extension the SDF, is displacing ethnic Arabs from northern Syria — decades after former Syrian President Hafez Assad settled them in what were once predominantly Kurdish areas.

A HISTORY OF MARGINALIZATION

The Kurds are a sizable ethnic group in the Middle East, inhabiting a territory stretching across what is now Turkey, Syria, Iraq and Iran. They number approximately 30 million across the region, including an estimated 2 million in Syria.

Kurds in all four countries have suffered from a history of marginalization, fueling aspirations for independence or autonomy. Syria's Kurds, who make up 10 percent of the country's population, were only granted the right to citizenship in 2011, with Assad seeking to win over the minority in the early days of the uprising against him.