The US on Wednesday called on the PYD, the Syrian branch of the southeast Turkey-based PKK terrorist group, to cease its support for the PKK.
Addressing Turkish concerns about links between the PYD and the PKK, Deputy Secretary of State Anthony Blinken said, "Turkey is one of our closest allies. Its collaboration in dealing with the crisis in Syria is absolutely vital and we are indeed working very closely together with Turkey.
"It is also not a secret that Turkey has expressed concerns about some of the Syrian Kurdish groups in northern Syria, including the PYD."
Speaking at a news conference at the UN in Geneva, Blinken added, "We also made it very clear to the PYD that any actions it takes to either support the PKK or to engage the other opposition groups are profoundly problematic and we look to the PYD to act responsibly and to focus its efforts on the fight against DAESH."
The PKK, a Marxist-Leninist group which seeks to impose its ideology on predominantly Kurdish regions, over the summer resumed its 32-year armed campaign against the Turkish state after unilaterally ending a two-year ceasefire. It is listed as a terrorist organisation by Turkey, the US and EU.
Turkey also considers the PYD and its YPG armed wing to be terror groups but the West has refused to designate them as such, instead choosing to see them as allies in the fight against the DAESH terrorist group in Syria.
Since establishing their authority along the Turkish border in northern Syria with the help of US-led coalition air strikes against DAESH, the PYD has been accused of committing a number of war crimes and atrocities against civilians, by both international and local groups.
UK-based rights group Amnesty International released a report in October which said that more than a dozen villages were destroyed by the PYD and that villagers were threatened with US-led air strikes if they failed to leave their homes.
A separate report published by the watchdog in September accused the PYD of using the fight against DAESH as a pretext for arbitrary detentions and unfair trials against "peaceful critics and civilians" in the regions that it controls.
Human Rights Watch also reported that the PYD were not meeting a commitment made in June 2014 to stop mobilising all recruits, both combatant and noncombatant, under the age of 18, citing evidence of 59 children, ten of whom were younger than 15, being recruited within the last year. Some of the recruits allegedly participated and died in battles in June 2015.