U.S. forces conducting patrols to deter aggression between Turkish forces and Kurdish elements of the Syrian Democratic Forces, both partners in the fight against the Islamic State, were photographed at a funeral April 29 where flags of the Kurdistan Workers' Party, or PKK, a U.S.-designated terror group, could be seen in the crowd.

For more than 30 years, the PKK has been involved in an armed struggle against the Turkish government in an attempt to secure self-determination for Kurds in Turkey.

The funeral in Derik, a small town in Syria's northeastern corner, was held for Syrian Kurd fighters of the People's Protection Units, or YPG, who were killed in Turkish airstrikes launched last week. The attacks also killed a number of Kurdish Peshmerga fighters. Both Kurdish groups are aligned with the U.S. effort to defeat ISIS. 

Officials in Baghdad say U.S. forces are operating in the region to reassure allies and partner forces of America's commitment to their security, while also observing and reporting security incidents near the border.

U.S military officials argue there was nothing nefarious to the presence of U.S. armored vehicles at the funeral. "They were just patrolling and nothing more to it than that," Col. Dorrian said. "I have not seen the images you are talking about. … [U.S. forces] were just performing their mission to patrol the area," he added.

The U.S. alliance with YPG  — which the Turkish government refers to as the PKK’s Syrian outshoot  — has strained relations between Ankara and Washington.

"Unfortunately... the presence of an American flag along with the (insignia) of a terror organization called YPG in a convoy has seriously saddened us," Turkish President Recep Tayyip Erdoğan told Agence France-Presseon Sunday. "This needs to be stopped right now."

The presence of PKK flags at the funeral is likely to further inflame the Turks.

The U.S. has come to rely on the Syrian Democratic Forces in its fight against ISIS, describing it as one of the most capable fighting forces in the region. The SDF is a loose alliance of fighters and organizations that includes the YPG fighters and Arab fighters with the Syrian Arab Coalition, Dorrian explained.

Turkey sees no difference between the YPG and PKK, however, U.S. military officials in Baghdad claim they "have observed no evidence" that YPG fighters are also fighting with or alongside the PKK.

Despite rising tensions in the region over the complicated partnership between the U.S. and its Kurdish allies, SDF forces made considerable progress to isolate the ISIS capital, Raqqa, over the weekend.

SDF forces, aided by coalition airpower, have captured the center of the Syrian city of Tabqa, the last remaining obstacle to surround Raqqa. Coalition airstrikes also destroyed 40 ISIS barges near Raqqa that were being used to move troops and supplies on the Euphrates River.