Turkey has for days been poised to unilaterally invade northern Syria over US objections, which Ankara officials say is to establish a 32 kilometer (20 mile) inside the war torn country, giving Turkey complete control of a region where the Syrian Kurdish YPG operates (People's Protection Units). Turkey has long considered the US-backed group, which forms the core of the Syrian Democratic Forces (SDF), to be a terrorist extension of the outlawed PKK.

The Pentagon has condemned the impending Turkish unilateral move, with US Defense Secretary Mark Esper telling reporters early Tuesday that it would be unacceptable and thwarted by Washington, though it's unclear how far the Pentagon would be willing to go. "What we're going to do is prevent unilateral incursions that would upset, again, these mutual interests that the United States, Turkey and the SDF share with regard to northern Syria," Esper said.

Crucially, according to ABC News, US officials "have made clear that an invasion is an extremely risky venture that could threaten the safety of U.S. forces working with the SDF...".

File image: Turkish tanks along the border with Syria.

On Sunday Turkish President Recep Tayyip Erdogan said that his forces would launch an operation in Syria east of the Euphrates River at an unspecified start date, and noted that the US and Russia had been notified.

In ongoing negotiations this summer the US and Turkey have clashed over just such a "safe zone," given Turkey wants the area completely clear of Kurdish armed groups, which the Pentagon simultaneously backs.

Turkish defense officials have lately threatened their "patience is limited" as the army builds up its forces along the border. The Foreign Ministry on Friday warned, "We won't let this process be dragged out. If our expectations aren't met, we are fully capable of taking whatever measures [are needed] to ensure our national security."

Ankara has long condemned US training, logistics support, and weapons going to the YPG, especially as it operates in Manbij, a key Syrian Kurdish stronghold near the border with Turkey.

US officials have been present in Ankara for talks early this week to try and negotiate a last minute settlement to avoid the invasion, with Esper noting Tuesday there's been "progress" made on certain key issues.