Turkey summoned the American ambassador a day after the U.S. Department of State indicated that Washington does not recognize the armed wing of the Kurdish Democratic Union Party (PYD) that controls large swathes of northern Syria as terrorists.

The Associated Press (AP) reports that Turkey summoned the U.S. ambassador to “convey unease” over its position on the People’s Protection Units (YPG), the main armed service of the PYD.

“A Foreign Ministry official said [U.S.] Ambassador John Bass was called to the ministry on Tuesday where a senior Turkish official expressed Ankara’s concerns,” notes AP. “The official spoke on condition of anonymity because she wasn’t authorized to make public statements.”

Turkey considers the U.S.-backed PYD a terrorist group that is affiliated with the Kurdistan Workers Party (PKK). The United States has officially designated the communist PKK group as a foreign terrorist organization.

On Monday, John Kirby, spokesman for the State Department, told reporters that the U.S. is aware of Turkey’s concerns over the PYD, but added that Syrian Kurds’ military wing is one of the most successful armed groups fighting the Islamic State (ISIS/ISIL) in Syria.

“We don’t, as you know, recognize the PYD as a terrorist organization,” declared Kirby. “We recognize that the Turks do, and I understand that. Even the best of friends aren’t going to agree on everything.”

“We have said that these Kurdish fighters are successful against Daesh,” he also said, referring to ISIS by its Arabic acronym, “these groups that we are providing support for, and we’re going to continue to provide that support because they are going after the group that we believe needs to be eradicated in Iraq and Syria.”

Kirby later described the YPG forces “as an important partner in this fight” against ISIS.

The spokesman acknowledged that Turkey is America’s NATO ally and a “key” member of the U.S.-led coalition against ISIS.

“Nothing’s going to change about the close relationship we have with Turkey and the efforts that we’re going to expend going forward to strengthen that relationship,” Kirby told reporters, noting that the United States has “seen no indications” that Ankara may diminish its cooperation against ISIS because of Washington’s position on the PYD and its armed component.

“We understand they have concerns about support to Kurdish fighters in Syria, again, these are longstanding concerns,” he also noted, later adding, “We’ve been managing this particular issue for quite some time. This is not a new concern, as I said, that the Turks have proffered.”

Nevertheless, the issue has seemingly strained the relationship between the two NATO allies.

Late last week, Turkish President Recep Tayyip Erdogan called on Washington to choose between Turkey and PYD as its partner.

In comments published Sunday, Erdogan reportedly asked, ”How can we trust you? Is it me that is your partner or is it the terrorists in Kobani?”

The Turkish president’s questions came a week after Brett McGurk, President Obama’s envoy to the anti-ISIS coalition, visited the border town of Kobani in northern Syrian that is under the control of the YPG.

According to the U.K.-based Syrian Observatory for Human Rights (SOHR), a group that uses a network of sources on the ground to monitor the Syrian conflict, McGurk “promised” the PYD officials he met with that a new Syrian constitution would recognize the Kurdish “right” to self-govern territory in the northern part of the country.

Retired U.S. Lt. Col. Sargis Sangari, who is currently advising the Christian Assyrian Army, told Breitbart News that, as it stands now, the region being promised to the Kurds includes all Christian Assyrian villages in Syria.

Sangari, a decorated Iraq war veteran who served 20 years in the U.S. Army, added that if the Syrian Kurds are granted autonomy, they will “eradicate the Assyrians in Syria and their Christian culture,” noting members of the YPG Kurdish militia have attacked a predominantly Assyrian area in northern Syria.

Kirby acknowledged that McGurk met Polat Chan, a YPG representative and former PKK militant.

Military Times reports that the State Department recently reiterated the longstanding U.S. policy that views the PKK as a terrorist group.

“We continue to call on the PKK to immediately cease its campaign of violence. A resumed political process offers the best hope for greater civil rights, security, and prosperity for all the citizens of Turkey,” reportedly said Noel Clay of the State Department.

In October 2015, the human rights NGO Amnesty International accused the YPG of committing war crimes.