BEIRUT (AP) — Backed by the United States, a multi-ethnic group known as the Syrian Democratic Forces is likely to lead the operation to capture the northern Syrian city of Raqqa, the Islamic State group's de facto capital.

The group has been the most effective and trusted force by the United States to battle IS in the northern and eastern parts of Syria. The SDF was predominantly Kurdish but over the past months, more Arabs have been recruited by the group ahead of the battle for Raqqa which has a predominantly Arab population.

Here's a look at the group:

WHAT IS THE SYRIAN DEMOCRATIC FORCES

A Kurdish-led alliance that includes Arab tribesmen and Christian fighters. The group which was founded in October 2015 has tens of thousands of fighters almost equally divided between Arabs and Kurds, according to Brig. Gen. Hussam al-Awwak, an Arab who heads public relations at SDF.

Cihan Sheikh Ehmed, an SDF spokeswoman, said the coalition has between 50,000 and 54,000 fighters, and more than half of them are Arabs.

With the help of American advisers and U.S.-led coalition airstrikes, the SDF has captured much of northern Syria and since November has been working to isolate the city of Raqqa. But the main Kurdish militia in the group is seen as a terrorist organization by Turkey, a NATO member which also has its sights on Raqqa.

The top U.S. commander in the campaign against IS, Lt. Gen. Stephen Townsend, said earlier this month that the SDF comprises 40 percent Kurdish fighters and some 60 percent are from the Syrian Arab coalition. Townsend said that of the Arab fighters in the SDF more than 3,000 have been trained by the Americans.

The group is likely to lead the battle into the northern city of Raqqa, the de facto capital of IS, in the coming weeks.

The SDF is now backed by hundreds of U.S. Marines and Rangers who will provide artillery fire support as they advance toward Raqqa.

The SDF controls nearly 19 percent of Syria's territory according to the Europe-based Center of Kurdish Studies.

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WHO ARE THE MAIN GROUPS AMONG THE SDF

The SDF consists of groups from different ethnic groups and religions who are mostly from northern and eastern Syria.

The main groups include the Kurdish People's Protection Units, or YPG and the Women's Protection Units, or YPJ. The coalition also includes predominantly Arab factions such as Army of the Revolutionaries, the Northern Sun Battalion, Euphrates Volcano, Raqqa Revolutionaries Brigade and Sanadid Forces. The Christian Syriac Military Council is also part of the SDF.

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WHO ARE THE MAIN BACKERS OF THE SDF

The U.S.-led coalition has been the main backer of the SDF, providing them with weapons and training.

The U.S. already has some 500 advisers working with the SDF in addition to a force of 400 that arrived more recently, according to Air Force Col. John Dorrian said. He said the troops include U.S. Army Rangers already in Syria.

French and German advisers are also known to be helping the SDF in Syria.