Turkey has hit out at the US over images said to show US special forces in Syria wearing insignia of Kurdish militia, during joint operations against so-called Islamic State (IS).
Turkish Foreign Minister Mevlut Cavusoglu called the US "two-faced" and said the practice was "unacceptable".
The images appear to show a US special forces soldier wearing the patch of the YPJ — a Kurdish militia group.
A Pentagon spokesman said troops often blended in with partners for safety.
The series of images, by an AFP photographer, show the US soldiers operating alongside members of the Syrian Democratic Forces (SDF), a US-backed anti-IS Kurdish-Arab alliance dominated by Kurdish fighters.
The photographer said the soldiers identified as US special forces refused to talk to journalists but were less wary than usual around the media.
The SDF is largely comprised of the YPG and YPJ, the men's and women's wings of the Kurdish People's Protection Units.
The US has publicly acknowledged that its troops are working alongside the YPG, but this is the first time images have emerged of apparent US troops wearing the militia insignia.
Turkish authorities accuse the YPG of being linked to the banned PKK Kurdish militant group, which the Turks, along with the EU and US, regard as a terrorist organisation.
The US military has refused to comment directly on the images but says it has about 300 troops in Syria in training and support roles but not participating in frontline combat.
Pentagon press secretary Peter Cook said it is common for US soldiers to attempt to blend in with local partners.
He said: "Special operations forces, when they operate in certain areas, do what they can to, if you will, blend in with the community to enhance their own protection, their own security".
The images were taken about 30 miles from the IS stronghold of Raqqa.
They emerged days after the SDF launched a military campaign to drive the jihadists back from territory north of the city.
The ground offensive is being supported by air strikes from the US-led anti-IS coalition.
Some observers were critical of the US forces' decision. Charles Lister, of the Middle East Institute, told AFP: "On a human level, I get it. They are probably doing it in some way to try and present [themselves] as a friendly ally to the locals.
"But the broader reality here is that US-Turkish relations are already on a pretty low level, and I know for a fact that something like this will have stirred significant anger in Ankara — and that's not a good thing."