A US-backed Kurdish militia has deployed fighters to the front line of Syria's Manbij to battle Turkey's military after President Recep Tayyip Erdogan said it would be next after launching the cross-border Afrin operation.
Manbij is about 100km east of Afrin where the United States has military personnel deployed in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), meaning troops from the NATO allies could come face to face on the battlefield.
The US military coalition operating in Manbij said soldiers there have the right defend themselves against any attack, and wouldn't hesitate to do so.
"Clearly we are very alert to what is happening, specifically in the area of Manbij because that is where our ... coalition forces are," spokesman Colonel Ryan Dillon told Reuters news agency. "The coalition forces that are in that area have an inherent right to defend themselves and will do so if necessary."
Sharfan Darwish of the Manbij Military Council - a unit of the YPG Syrian-Kurd militia currently under attack by Turkey in Afrin - said his forces were preparing to confront Turkish soldiers.
"We are in full readiness to respond to any attack. Of course our coordination with the international coalition continues with regards to the protection of Manbij," said Darwish.