The US has expressed concern over the attitude of the YPG, the armed wing of PYD, towards US backed opposition forces in Syria.
State Department spokesman Mark Toner said on Thursday that the US wants the YPG to focus its fight on DAESH without becoming involved in any other fighting in Syria.
"We've expressed our concerns, frankly, when they come into conflict with opposition forces on the ground," Toner said.
"We feel that sets back the overall effort, what we're trying to achieve on the ground," he added.
Continuing his words, Toner said that US is continuing to urge the YPG to "administer inclusively on territory that they've liberated, protect local populations and property and promote and protect human rights."
Toner’s comment came after reports over that YPG fought with moderate Syrian opposition forces in the Azaz corridor, which is a narrow belt in the west of Euphrates river that runs from the Turkish border to Aleppo.
Turkey lists the PYD and its armed wing YPG, as terror organisations.
The YPG has recently been supported by Russian air power in northern Syria.
The PYD recently opened an office in Moscow adorned with a poster of PKK leader Abdullah Ocalan.