Turkey has summoned Washington's ambassador to Ankara Jeff Flake to protest against the recent visit by Gen Mark Milley, chairman of the US Joint Chiefs of Staff, to Kurdish-controlled territories in Syria.
State Department spokesman Ned Price confirmed on Monday evening that Mr Flake went to the Turkish Foreign Ministry for discussions.
Gen Milley made an unannounced visit on Saturday to review a nearly eight-year-old US mission to an area controlled by the US-allied Syrian Democratic Forces.
Turkey considers SDF spearhead YPG, or the People's Protection Units, to be the Syrian wing of the outlawed separatist Kurdistan Workers' Party, or PKK, and has designated both as terrorist organisations.
Mr Price told reporters it was the State Department's understanding that Gen Milley only met US troops while in Syria and referred the media to the Department of Defence for details.
The SDF, the primary ally of the US-led coalition against ISIS, played a key role in defeating ISIS fighters across Syria.
However, Washington's support for the group has been a source of tension with Ankara for years.
The US and the EU have also designated the PKK as a terrorist group, but not the YPG.
Analysts and military experts say ISIS remains a threat years after its territorial defeat in 2019.
US troops remain stationed in northern Syria, providing the SDF with intelligence and training in the Kurdish group's fight against ISIS.
“An enduring defeat of ISIS and continuing to support our friends and allies in the region … I think those are important tasks that can be done,” Gen Milley said during his trip.
