Turkey wants no confrontation with U.S. troops, but said they risk being caught up in clashes if they wear YPG uniforms.
Syria's Kurdish militia is growing frustrated with its patron, the United States.
Turkey renewed threats over the weekend to attack a town in northern Syria where U.S. troops maintain an outpost.
Over the weekend Turkish forces launched an incursion into Afrin, a border canton defended by the YPG militia.
Intense fighting flared Monday as Turkish troops and their allies advanced on a Kurdish enclave in northwestern Syria.
Syria's peace efforts cannot include President Bashar Assad, Turkey's leader said Wednesday.
"Anywhere you look across all the Syrian fault lines, Moscow has positioned itself as a key player."
Syria is awash in weapons and an array of local militias and thousands of foreign troops, some of which may not leave.
Trump told Turkey's President Erdogan that he'd "given clear instructions" that the Kurds will receive no more weapons.
The incident happened during a NATO military exercise in Norway and resulted in Turkey withdrawing its 40 troops last week.