The US has relied heavily on the latter - especially the Syrian Kurdish People's Protection Units (YPG), the main component in the SDF - in its fight against Islamic State (IS) in Syria. It has even trusted the SDF as its partner on the ground in the offensive to capture Raqqa, which the SDF launched on 6 November.
The US supported Turkey's Euphrates Shield operation in Syria shortly after it began on 24 August. However, it has refused to support Turkey's attacks on SDF/YPG targets, instead invariably calling on both sides to deescalate clashes and focus on fighting IS.
On 8 December, the spokesman for the US coalition against IS, John Dorrian reiterated this US stance, saying that the US is currently "facilitating joint discussions with Turkey, the SDF and other coalition partners to promote deescalation".
Washington was caught off guard when Turkey launched Euphrates Shield, but instantly rushed in to provide Turkey and its Free Syrian Army (FSA) allies on the ground air support. They even briefly sent in a small number of special forces. This support was short-lived in light of Turkish clashes with the YPG, its advances further south of the border to al-Bab, and its vow to then move to capture Manbij from the SDF/YPG.
"Turkey started the Jarablus operation unilaterally," an unnamed US official told Turkey's Hurriyet news on 9 December. "Then after an agreement we reached to clear the border area from IS and to create a buffer zone, we supported the operation."
|Two important developments have taken place this month which indicate Washington is taking steps to try and reassure both its Turkish and SDF allies|
"But when Turkey advanced south from Dabiq," the official added, "it was different than what we agreed, and the coalition didn't support these efforts".
This may be set to change. Two important developments have taken place this month which indicate Washington is taking steps to try and reassure both its Turkish and SDF allies. One was the Obama administration's waiving of the US Arms Export Control Act on 8 December.
This will allow the US to provide "defense articles and services to foreign forces, irregular forces, groups, or individuals engaged in supporting or facilitating ongoing US military operations to counter terrorism in Syria". A move the White House has determined "is essential to the national security interests of the United States".
This justification echoes a statement by US Army Lt. General Stephen Townsend on October 26, warning there is an IS terror plot "emanating from Raqqa".
"We think we've got to get to Raqqa pretty soon," he added, less than two weeks before the SDF launched their aforementioned offensive.
Turkish President Recep Tayyip Erdogan argued that the US and Turkey could take Raqqa without including the SDF/YPG, insisting that the YPG is a terrorist organisation as bad as IS.
In a clear move to placate the Turks, as they move to arm the SDF and support their march on Raqqa, US Central Command announced on its official Twitter account on 10 December that 203 US special operations forces "will in part directly assist NATO ally Turkey in the counter-ISIL fight".
Having US backing against IS in northwest Syria could prove invaluable to Ankara at this time. The Syrian regime staunchly opposes a Turkish-backed FSA takeover of al-Bab.
A mysterious air attack that killed four Turkish soldiers outside al-Bab on 24 November was initially blamed by Ankara on Damascus.
|Turkey may well acquiesce to the SDF's Raqqa offensive in return for such important US support in al-Bab|
The SDF warned in a 20 November statement that Turkish air and artillery attacks against the SDF's Manbij Military Council could lead to a wider escalation.
"In the event of attacks against the region continuing, we will intervene and fulfil our role on supporting the forces of the Manbij Military Council," the SDF declared. "We state that we are ready to support the Manbij Military Council and respond to the occupation attacks if attacks from the Turkish army and backed gangs are to continue."
Such an escalation could result in the SDF abandoning its Raqqa campaign and focusing instead on fighting Turkey and its allies, which would be a disaster for Washington's current plan in Syria.
To prevent such an undesirable outcome the US is likely increasing its support to both parties provided they focus on fighting IS rather than each other.
Paul Iddon is a freelance journalist based in Erbil, Iraqi Kurdistan who writes about Middle East affairs.
Follow him on Twitter: @pauliddon