On Monday the Kurdish-led Syrian Democratic Forces (SDF) – who occupy former IS territory in the north-east of Syria – said US forces had begun to withdraw from border areas, accusing them of US failing to “fulfil their responsibilities”.
An SDF spokesman said the group was “determined to defend NE Syria at all costs”.
The US says it is stepping aside for an imminent Turkish operation against Kurdish-led forces within Syria that have until now been a key US ally.
Kurdish militias played a major role in defeating the Islamic State (IS) group, but Turkey regards them as terrorists.
The US – which has hundreds of troops near the Turkey-Syria border – has reportedly begun to withdraw them.
The main Kurdish-led force has condemned the pullout and accused the US of abandoning its fighters.
The Syrian Observatory for Human Rights, a UK-based monitoring group, also said US forces had pulled back from key positions.
Kurdish TV in northern Iraq said the SDF had put some of its units on alert because the Turkish army had mobilised troops on the border on Monday.
The YPG was a major part of the SDF, the US-supported force that defeated IS in Syria.
Turkey considers the YPG an extension of the banned Kurdistan Workers’ Party (PKK) which has fought for Kurdish autonomy in Turkey for three decades.
The YPG denies any direct organisational links to the PKK.
Turkey has previously condemned the US for supporting the YPG.
