In late 2015, elements of the PYD -- the PKK’s Syrian arm -- and small groups of Syrian opposition fighters came together to form the SDF.

Since then, fighters from the Syrian Arab Coalition, the Army of Revolutionaries, the Al-Jazeera Brigades and the Syriac Military Council -- among other groups -- have all joined the SDF, which continues to enjoy U.S. support.

The SDF is reportedly drawn up largely of PYD/PKK elements who administer and supervise the group’s operations.

The U.S. administration, for its part, has said from the outset that it was providing support to the SDF and not to elements of the PYD/PKK.

The SDF now seeks to cement its control over northern Syria. To this end, it has enjoyed frequent U.S. air support, with which it managed to expand to the west of the Euphrates in late 2015.

In a report published Jan. 7, the Washington Post asserted that PYD/PKK elements accounted for a full one third of the SDF’s current fighting force.

According to the newspaper, these elements seek to promote the ideology of jailed PKK leader Abdullah Ocalan in SDF-held areas.

The Obama administration, for its part, considers the PYD/PKK one of its most important allies in the fight against ISIL, describing it as "moderate" and "reliable".

The U.S., however, has refrained from providing the same support to legitimate Syrian opposition groups that are not considered "terror organizations" by Washington.

Several Arab members have left the SDF in recent days after complaining that they had been marginalized from the group’s decision-making process.

Under the guise of the SDF, the PYD/PKK has managed to establish its influence over several parts of northern Syria adjacent to the border with Turkey.

These areas include Al-Hasakah province, Ain al-Arab (also known as Kobani) and Afrin to the west of the Euphrates River.

The group now seeks to assert its control over Al-Bab to the southwest of Manbij with a view to linking the areas under its influence on either side of the river.

Thus far, however, Operation Euphrates Shield -- launched by the Turkish military last year in coordination with the Free Syrian Army -- has prevented the SDF from gaining a foothold in Al-Bab.