Turkey calls them "terrorists". The US sees them as allies against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).
Kurdish forces in Syria have been working with the US army for months now. But Turkey found out this week that American troops have been wearing the Kurdish group's insignia.
And they are not happy about it. US troops say they were wearing it to blend in.
The YPG is an armed group made up mostly of Syrian Kurds. And they are one of the most effective groups fighting against ISIL in Syria right now.
But Turkey says it's an extension of the banned PKK, that has been fighting for Kurdish autonomy for three decades.
So, how will this spat between Turkey and the US impact the ongoing war in Syria?
Presenter: Martine Dennis
Metin Gurcan, former Turkish military officer and a columnist for Al-Monitor.
Rebwar Fatah, founder and managing director of Middle East Consultancy.
Mike Lyons, senior fellow at the Truman National Security Project.
Source: Al Jazeera