A delegation including senior US diplomat, Brett McGurk, met members of Syrian Democratic Forces (SDF) alliance, local sources said on Sunday.
The weekend visit to the war-torn country -confirmed by a US official- appeared to be the first by a senior US government figure inside Syrian territory.
McGurk, who is US President Barack Obama's envoy to an international coalition fighting DAESH in Syria and Iraq, was accompanied by French and British officials, the sources told AFP.
SDF is an armed group formed in late 2015 operating in northern Syria.
It is composed mostly of the YPG, along with smaller units of Syrian Arab Muslim and Christian fighters.
YPG is the armed wing of PYD, which is considered by Turkey as the Syrian wing of PKK, a terrorist organisation according to Turkey, the US and the EU.
"These meetings will have an impact on many developments that will be seen in the area," the source told AFP, without providing further details.
A US official said that McGurk visited Syria at the weekend to take stock of the campaign against DAESH, but gave few other details.
The meetings come after the PYD was excluded from UN brokered peace talks in Geneva, following objections from main Syrian opposition groups and Turkey saying PYD has close ties with the Bashar al Assad regime and collaborates with it.