The US's main partner on the ground in Syria continue to be the Syrian Democratic Forces (SDF). They includes the Kurdish YPG militia, which Turkey views as an extension of the PKK militant group, seen as a terrorist group by Ankara.
The PKK has waged a bloody three-decade insurgency in Turkey, trying to force the Turkish government to allow the creation of a separate Kurdish state. It's a proposal which Turkish President Erdogan utterly rejects.
The inclusion of the YPG in the US-backed SDF has therefore been a major point of contention between the US and Turkey.
This was inflamed on Wednesday January 11, when the US Central Command tweeted a statement validating their support for the SDF.
SDF confirms that it has no affiliation or ties to PKK pic.twitter.com/mvENTcBpoQ— U.S. Central Command (@CENTCOM) January 11, 2017
Is this a joke or @CENTCOM has lost its senses? Do you believe anyone will buy this? The US must stop trying to legitimize a terrorist group— Ibrahim Kalin (@ikalin1) January 12, 2017
It seems a significant amount of global diplomacy is now being conducted via Twitter.
The rift between Turkey and it's NATO ally, the US is growing increasingly frosty. Political analysts note that this widening divide risks undermining their mutual goal-the fight to drive out Daesh from Syria.
The fissure was already widened earlier in the week when on January 10, US State Department deputy spokesman Mark Toner, said that the YPG's political wing, the Democratic Unity Party (PYD), should be allowed to take part in the peace talks on Syria.
"This process has to include all Syrians, and that includes the Syrian Kurds," Toner said.
The statement was rejected by Ankara. No wonder, as since August 2016, Turkey, and the rebels they support, have been fighting the YPG, in addition to Daesh.
On January 7, the Turkish army announced that it had killed 291 YPG fighters.
The Turkish forces next target is Al-Bab. Just 20 miles south of the Turkey-Syria border, it is strategically important to Turkey because Kurdish-dominated militias are also attempting to control it.
Tellingly, the US has declined to offer support for Turkey's Al-bab campaign.
The Turkish government has expressed hope of a "new chapter" of relations with President-elect Donald Trump. However, it's not yet clear whether the new administration intends to continue with or end its support for the YPG.