It started two weeks ago, when Turkey warned publicly it was preparing for military intervention in Syria, while accusing the US of creating a "terrorist army" (it wasn't referring to ISIS, but US-backed Syrian Kurdish militia YPG). As a reminder, YPG forms a major part of the U.S.-backed campaign to capture Islamic State's stronghold of Raqqa, and whose forces are seen as a terrorist organization by Turkey. The group currently controls a pocket of territory in Afrin, about 200 km (125 miles) west of Raqqa.

Tensions between Turkish forces and the YPG have been mounting in the Afrin region in recent weeks: Turkey's military, which launched an incursion last August into part of northern Syria which lies between Afrin and a larger Kurdish-controlled area further east, has said that it has returned fire against members of YPG militia near Afrin several times in the last few weeks.

Furthermore, last month the Turkish defence ministry slammed the Pentagon decision to arm theYPF, and mocking Washington's assurances that it would retrieve weapons provided to the YPG after Islamic State fighters were defeated: "There has never been an incident where a group in the Middle East has been armed, and they returned the weapons," Kurtulmus said. The United States "have formed more than a terrorist organisation there, they formed a small-scale army."

Then overnight, Ilnur Cevik, a senior adviser to President Recep Tayyip Erdogan, spoke to Bloomberg and said that while Turkey has no immediate plans for an operation in the Syrian Kurdish-run region of Afrin, its army is preparing for action and the military buildup on the border is “serious."

“PYD militants are also harassing the people of Idlib to the south and the people in the areas which Turkey liberated from Daesh,” Cevik says and alleged that PYD’s goal is to “dislodge Turkey from areas that the Free Syrian Army and Turkey liberated,” making their actions “an elevated threat.”

“They want to create a string of cantons in northern Syria and they feel the areas that were liberated with the Euphrates Shield operation should be theirs”; says group wants to “throw Turkey out of there and connect all their cantons in northern Syria and create a mini state. It’s not only Afrin, the threat comes from Kobani, the threat comes from Qamishli, the areas to the east of the Euphrates river until the Iraqi border from areas controlled by the PYD/YPG. Turkey feels threatened from that area as well but the immediate concern is Afrin.”

He concluded by cryptically saing that while “Turkey does not have immediate plans to enter Afrin but Turkey feels that sooner or later we have to do something. This buildup is necessary. And the president said that we are not going give a date or anything, but suddenly one night we may do something there.”

Or perhaps during the day, because according to Rudaw, just hours after Celik's interview, Turkey commenced bombing YPG positions in Azaz, roughly 5 miles northeast of Afrin in north east Syria, and in immediate proximity to the Turkish border.

Separately Conflict News reported that on Monday morning there has been heavy Turkish artillery shelling of US-backed Syrian YPG/SDF forces in the region:

So far there has been no response from either the US or NATO, to what appears to be increasing hostilities between NATO member Turkey and US-ally YPG.