US troops in northern Syria came under direct attack last week by Turkish-backed rebels, a military official with the coalition fighting ISIS told CNN Tuesday. The official said that while US troops returned fire there were no casualties on either side.

The coalition believes the attackers are part of the Turkish-backed opposition forces, a loose grouping of Arab and Turkmen fighters that have helped the Turkish military clear ISIS from the Turkish-Syria border area.

“Recent incidents have occurred in territories primarily under the control of Turkish-backed fighters. We are engaged with Ankara and other parties to address this danger,” Pentagon spokesman Eric Pahon told CNN.

Many of these Turkey-supported forces originated as part of the opposition to Syrian President Bashar al-Assad and they have also clashed with the US-backed and Kurdish-led Syrian Democratic Forces in a struggle for influence in the region.

While Turkey has in the past backed these fighters with tanks, airstrikes and special forces, the coalition official made it clear that Turkish soldiers were not involved in the recent attacks on US personnel.

But the official added that the coalition had delivered a demarche, or diplomatic protest, to Ankara following the attack on US forces by Turkey’s allies.

The incident occurred around the same time US Secretary of Defense Jim Mattis visited Turkey to meet with his Turkish counterpart and the country’s President Recep Erdogan. There was no mention of any conversations about the incident in the official readout of the meeting provided by the Pentagon.

The recent clash comes weeks after attacks against US troops in the area were first reported and confirmed by coalition spokesman Col. Ryan Dillon. Following those attacks, Dillon said US and coalition troops “reserve the right to defend ourselves.”

US forces have been in northern Syria for months where they are performing a de facto “peacekeeping” role in an effort to prevent clashes between various armed groups in the region. While Pentagon spokesman US Navy Capt. Jeff Davis declined to call the troops “peacekeepers” upon their initial deployment in March, he referred to them as “a visible reminder for anybody looking to start a fight.”

Dillon said at the time that the attacking forces were most likely aware that they were firing on US troops.

“These patrols are overt. Our forces are clearly marked and we have been operating in that area for some time,” Dillon said, adding, “It should not be news to anyone that we are doing this, operating in that particular area.”

US troops have been performing “overt patrols” in the area since March, often flying the American flag from armored vehicles, in a bid to deter forces in the region from attacking one another and undermining the fight against ISIS.

The US trains and advises the Manbij Military Council, a group of local Arab fighters that is allied to the Kurdish-led Syrian Democratic Forces. But Turkey and its local Syrian allies are strongly opposed to both US-backed groups which they see as linked to Kurdish separatists in Turkey.

“Coalition troops will continue performing patrols within the Manbij Military Council area of control,” Pahon, the Pentagon spokesman said, adding “Coalition forces are there to monitor, deter hostilities and ensure all parties remain focused on our common enemy and the greatest threat to regional and world security, ISIS.”

“We take appropriate measures to try and minimize the risk of those events happening. But they do happen, and of course, all coalition forces have the inherent right of self-defense at their disposal, should they feel the need,” the Deputy Commander of the counter ISIS coalition, UK Army Maj. Gen. Rupert Jones, told reporters at the Pentagon last week.

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