Images have emerged appearing to show US commandos operating with Kurdish and Arab fighters near the frontline of an offensive against so-called Islamic State (IS) militants in northern Syria.
In some, soldiers identified as US special forces by the AFP photographer are seen wearing the patch of the local Kurdish fighters.
About 300 US special forces have been deployed to northern Syria.
The US says their function is to train and assist rebels in a non-combat role.
The YPG (Kurdish People’s Protection Units) and the YPJ (Women’s Protection Units) comprise the majority of the US-backed Syrian Democratic Forces (SDF), an anti-IS alliance.
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.
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 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 could further anger the Turkish government, which is already deeply unhappy at US support for Kurdish groups inside Syria.
Turkish authorities accuse the groups of being linked to the banned PKK Kurdish militant group, who the Turks, along with the EU and US, regard as a terrorist organisation.
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.”