The Kurdish news outlet Rudaw and Turkish network Haberturk are reporting that members of the Syrian Kurdish Peoples’ Protection Units (YPG/YPJ) and the Turkish military have exchanged fire in northwest Syria.

Both the YPG and the Turkish government claim to be fighting the Islamic State in Syria. The Turkish government considers the YPG a terrorist group, however, and has repeatedly demanded it cease operations in the area. The YPG is an ally of the United States and has provided America intelligence for airstrikes against ISIS for years. In May, the Pentagon announced that it would provide the YPG/YPJ with weapons as necessary to ensure a victory over ISIS in the battle for Raqqa, the Islamic State “capital.”

Rudaw cites the Syrian Observatory for Human Rights as reporting that the Turkish military has “shelled repeatedly” targets in Afrin, northern Syria. Rudaw received confirmation from the YPG that Turkey’s troops had “increased their attacks against Rojava,” or Syrian Kurdistan, claiming the Turks had been attacking Kurdish targets “daily.”

Reuters reported Tuesday, translating the information from Haberturk, that “military sources” had confirmed that Turkey and the YPG had exchanged fire in the regime. “Turkish officials could not immediately be reached for comment on the report, which said Turkey’s military retaliated after YPG fighters opened fire on their positions,” Reuters stated.

The Syrian outlet al-Masdar, a pro-Assad publication, independently published a report claiming that Turkish troops and the YPG are fighting each other, adding that “Turkey hopes to link the Idlib rebel heartland with the northern Aleppo pocket in order to strengthen the Syrian Opposition at the expense of the Syrian Democratic Forces (SDF).” Turkish President Recep Tayyip Erdogan has stated the Turkish troops are in Syria to “end the rule of the tyrant Assad,” though he later backtracked on that sentiment.

The Turkish government has repeatedly condemned American cooperation with the YPG, claiming that it is indistinguishable from the Kurdistan Workers’ Party (PKK). The PKK is a U.S.-designated Marxist terrorist group that, according to a human trafficking report published by the U.S. State Department Tuesday, employs the use of child fighters.

The United States treats the YPG and the PKK as separate entities. In its announcement in May that the United States would provide weapons for the YPG, spokesperson Dana White described them and the Syrian Defense Forces (SDF), a Kurdish-led multi-ethnic coalition, as “the only force on the ground that can successfully seize Raqqa in the near future.”

As of this Monday, reports suggest that the SDF have largely managed to successfully encircle the city of Raqqa, but the urban battle to rid the city of ISIS terrorists will be arduous. “In the first two weeks [of the campaign], there was significant progress—very quick progress that was made… the SDF in the advance have since hit some significant resistance from ISIS,” U.S. coalition spokesman Col. Ryan Dillon said on Sunday.

The Turkish government, meanwhile, has floated the claim that Secretary of Defense James Mattis promised to take back any weapons provided to the YPG following the liberation of Raqqa. Commenting on Ankara’s claims Tuesday, Mattis said, “we’ll do what we can.”

“We’ll see. It depends what the next mission is. I mean, it’s not like the fight’s over when Raqqa’s over,” Mattis asserted.

Mattis is scheduled to meet his Turkish counterpart, Defense Minister Fikri Işık, in Brussels on Wednesday, where both have arrived to attend a NATO summit.