Photos of US troops wearing patches from the Kurdish People's Protection Unit, known as the YPG, while fighting the Islamic State alongside Kurds in Syria have enraged Turkey's foreign minister, who on Friday called the photos "unacceptable."

"It is unacceptable that an ally country is using the YPG insignia," the foreign minister, Mevlut Cavusoglu, said, according to the Turkish daily newspaper Hurriyet. "We reacted to it. It is impossible to accept it. This is a double standard and hypocrisy."

The photos, taken by Delil Souleiman for AFP, have reignited the debate over Washington's support for the YPG, with some calling the patches "politically tone deaf" and others insisting it is "perfectly normal."

About 250 US special-operations troops were sent to northern Syria earlier this year to advise Kurdish and Arab forces battling the Islamic State there. The YPG has proved to be the most effective ground force fighting the Islamic State, the militant group also known as ISIS, ISIL, or Daesh, but the territorial expansion the YPG's victories have afforded it is vehemently opposed by Turkey, an important US ally and NATO member.

A close-up of the YPJ patch worn by a US service member in Syria. The YPJ is the all-female brigade of the YPG. AFP

Ankara views Kurdish demands for autonomy as a threat to Turkey's sovereignty and backs many of the rebel groups that have clashed with the YPG. Turkey has also linked the YPG to the Kurdistan Workers' Party, the designated terrorist organization known as the PKK that is waging an insurgency in Turkey's southeast.

Some analysts therefore speculated when the photos first emerged Thursday that the Americans' show of solidarity with the Kurds would further inflame tensions between the US and Turkey.

As one Kurdish activist asked on Twitter, "How will Erdogan react?"

Charles Lister, a Syria expert and senior fellow at the Middle East Institute, said it was "absolutely remarkable seeing US special forces personnel wearing YPG patches in the northern Raqqa operation."

"The US National Counterterrorism Center (NCTC) labeled the YPG the Syria wing of the 'designated' PKK in 2014," he added.

Michael Weiss, a Middle East analyst and coauthor of "ISIS: Inside the Army of Terror," noted on Twitter that the image on the YPG patch appeared to derive from the original PKK flag.

Pentagon spokesman Peter Cook told reporters in a press briefing on Thursday that the "special-operations forces, when they operate in certain areas, do what they can to blend in with the community to enhance their own protection, their own security."

He would not comment on the specific photos but added that the troops were most likely just being "supportive of that local force [YPG] in their advice and assist role."

Turkey's foreign minister shot back: "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 added: "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."

Wladimir van Wilgenburg, a field researcher for the Iraqi Institute for Strategic Studies and a journalist based in the region, said the practice was actually "quite normal."

"They do it out of respect for the local forces they are working with," van Wilgenburg told Business Insider on Thursday. "It's the same with coalition soldiers in Iraqi Kurdistan. I have seen them with Kurdish flags, or patches of different peshmerga forces (like the Zerevani)." A man with a flag of YPG, a Syria-based Kurdish militant group, during a protest against Turkish President Recep Tayyip Erdogan in front of Brookings Institution in Washington on March 31. Sait Serkan Gurbuz/AP

He added: "It has nothing to do with politics. They are fighting together as a 'band of brothers' against the Islamic state, so it's quite normal."

But Emile Hokayem, a Middle East analyst at the International Institute for Strategic Studies, tweeted that the photos were "politically tone-deaf and counterproductive in this context." He was most likely referring not only to the US-Turkey relationship but also to the tension between Kurdish forces and Syrian Arab rebel groups associated with the Free Syrian Army.

Mutual distrust continues to cast a shadow over the Kurdish-Arab relationship in northern Syria, even as the Obama administration has tried to bring Arab and Kurdish forces together via the Syrian Democratic Forces to fight the Islamic State.

FSA rebels were reportedly enraged, for example, when they learned that the US's top military commander, Gen. Joseph Votel, visited Kurdish commanders in northern Syria last weekend to discuss the Kurdish-dominated SDF's plans to retake territory from ISIS.

Syria Democratic Forces fighters on the outskirts of al-Shadadi in Syria's Hasaka countryside. REUTERS/Rodi Said State Department spokesman Mark Toner acknowledged the tensions in a news conference on Thursday, telling reporters that the Pentagon recently graduated a class of 200 Arabs to join the SDF.

"We're cognizant of the need to have diversified forces conducting these kinds of operations, given the sensitivities of the communities that they're liberating," Toner said.

Still, many FSA groups don't trust the Kurds, who wish to carve out an autonomous region in northern Syria known as Rojava, and are wary of US support for them.

"The Arab fighters [in the SDF] are just camouflage," Gen. Salim Idris, the former FSA chief of staff, told Voice of America last week. "The SDF is the YPG, which collaborates with anyone — Assad, the Russians, the Americans — when it suits its purposes."

He added: "I really don't think the Obama administration has thought this through. Will the Kurds give up Arab towns they capture?"

Note: This post has been updated to include comments from Turkey's foreign minister, Mevlut Cavusoglu.