Turkey on Friday lashed out at the United States after images surfaced apparently showing U.S. soldiers in Syria wearing the insignia of a Kurdish group opposed by Ankara.

Foreign Minister Mevlut Cavusoglu said during an international conference in southern Turkey that it was "unacceptable" for soldiers of a Turkish ally to use the patches of the YPG — the Kurdish People's Protection Units which are fighting the Islamic State group in northern Syria.

He said Turkey had relayed its displeasure to U.S. officials in Washington and in Turkey, and rejected explanations that the patches were for the soldier's protection.

"In that case, we would recommend they use the patches of Daesh, al-Nusra and al-Qaida when they go to other parts of Syria and of Boko Haram when they go to Africa," he said in reference to Jihadi extremist groups.

"To those who say they don't consider the YPG to be the same as these terrorist groups, this is our response: this is applying double standards, this is being two-faced," Cavusoglu added.

Turkey views the YPG as an extension of the Kurdish Workers' Party, or PKK, an armed insurgent group which it has fought for decades and is considered a terror organization by Turkey, the United States and the European Union.

Turkey is an ally in the U.S.-led coalition against IS but the two countries are at odds over the involvement of Kurdish fighters in northern Syria in the battle against the group.

On Thursday, Pentagon Press Secretary Peter Cook said U.S. special forces have in the past "worn insignias and other identifying marks with some of their partner forces."

"What I will say is that 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," he said.

Cook also faced a barrage of questions about whether or not the special operations forces were on the front lines, since in the past U.S. officials have stressed that American troops are not at the battlefront.

"We do have special operations forces in Syria," Cook said, adding that they are an "advise and assist mission" with forces that carry out the fight against IS and they "are not at the front line."

He declined to provide any specifics about their location, but reports suggest they were in or near Fatisah, which was one of three villages that Syrian forces took back from Islamic State militants in recent days.

Senior military officials said that it is difficult to tell where exactly the forces were, and how far from the fighting they were. They said the troops were moving with Syrian rebel forces as they headed toward Raqqa, and that it's possible they were closer to the front line of battle than they had been before. But they said there has been no change to the policy that the commandos will advise and assist but not fight on the front lines.

The officials were speaking on condition of anonymity because they weren't authorized to discuss the matter publicly.

There has been about 50 special operations forces in Syria doing the advising mission. But President Barack Obama has authorized the deployment of 250 more. Officials would not say exactly how many are currently in Syria, but said that all of the additional 250 have not arrived.

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Baldor reported from Washington D.C.