The US announced plans on Sunday for a "border security force" - made up of Kurdish and Arab fighters - to prevent a resurgence by ISIL.
The BSF will be tasked with securing the long sections of Syria's northern border with Turkey and eastern border with Iraq that are under the fighters' control, as well as parts of the Euphrates river valley, which effectively serves as the dividing line between them and Syrian pro-government forces.
Ankara has repeatedly warned Washington over its support for the Syrian Kurdish People’s Defence Units (YPG), which it sees as terrorist group over its links to the Kurdistan Workers’ Party (PKK) waging a bloody insurgency in southern Turkey.
But reacting to the news on Monday, President Recep Tayyip Erdogan could barely contain his rage.
"A country we call an ally is insisting on forming a terror army on our borders," he said of the US in a speech in Ankara. "What can that terror army target but Turkey? Our mission is to strangle it before it's even born.
"Don't stand between us and these herd of murderers. Otherwise, we won't be responsible for the unwanted incidents that may arise," he continued.
He said that the Turkish army was ready to launch an operation against the YPG in the northern Syrian enclave Afrin in the coming days.
The Turkish army had already positioned a convoy of tanks and was pounding the area yesterday with artillery from its positions inside Syria.
The YPG was the backbone of the fight against Isil in Raqqa. Mr Erdogan tolerated the US’s backing of Kurdish groups during the operation to liberate the city, in the hope Washington would abandon them after the city was liberated.
But the latest plans hint at the US’s longer-term plans for their involvement in the region and will concern Turkey, which fears the move is a step towards Syrian Kurds achieving a breakaway state.
The threat could bring the two Nato allies, who once worked together to support rebel groups opposed to President Bashar al-Assad’s government, into direct military confrontation.
“The Kurdish people will rise up as a whole. It will be total warfare,” Saleh Muslim, former head of the YPG's political wing, said in a warning to Turkey.
Source: Telegraph