The US is to stop supplying arms to the Syrian Kurdish militia the YPG, Turkey has said.
Foreign Minister Mevlut Cavusoglu said President Donald Trump had made the promise in a phone call to his Turkish counterpart, Recep Tayyip Erdogan.
The White House said it was making "adjustments" to its support for partners inside Syria but did not explicitly name the YPG.
Turkey has long complained about US support for the group.
Washington has viewed the YPG as a key player in the fight against so-called Islamic State (IS), but Ankara brands the group's fighters as terrorists.
Turkey says the YPG is as an extension of the outlawed Kurdistan Workers Party (PKK), a group it has been fighting for decades in south-eastern Turkey.
The US, however, has seen the YPG as distinct from the PKK. In May it announced it would supply arms to the Kurdish elements of the Syrian Democratic Forces (SDF), which were poised to drive IS from its stronghold of Raqqa.It had previously armed only Arab elements of the SDF.
"President Trump instructed [his generals] in a very open way that the YPG will no longer be given weapons," Mr Cavusoglu was quoted as saying in the Turkish Hurriyet Daily News.
The SDF, an alliance of Kurdish and Arab militias, has driven IS militants from much of the land it once controlled.
The SDF declared victory in Raqqa last month after a four-month battle to retake the city from IS, which had ruled it for three years.