The Pentagon said Thursday the U.S. military shot down an armed Turkish drone that came within 500 meters (yards) of American troops in northeastern Syria, in a rare use of force by one NATO member against another.

Air Force Brig. Gen. Patrick Ryder, the Pentagon press secretary, called it a “regrettable incident” and said U.S. troops were forced to go to bunkers for safety as Turkey bombed targets nearby, the Associated Press reported.

Both Defense Secretary Lloyd Austin and the new Joint Chiefs chairman, Gen. CQ Brown, spoke with their Turkish counterparts quickly after the incident to emphasize the value they place on their relationship with Turkey — but also the need to avoid any similar incidents in the future and ensure the safety of U.S. personnel.

The decision to shoot down an ally’s armed drone “was made out due diligence and the inherent right of self-defense to take appropriate action to protect U.S. forces,” Ryder said, adding that “we have no indication that Turkey was intentionally targeting U.S. forces.”

U.S. officials earlier told The Associated Press the shootdown was ordered after more than a dozen calls to Turkish military officials stating that U.S. forces were on the ground in the area and that the U.S. military would take action to protect them if the drone didn’t leave. The officials spoke on condition of anonymity to provide details of a sensitive military incident.

Ryder said U.S. forces observed Turkish drones doing airstrikes around Hassakeh at about 7:30 a.m. local time, and some strikes were inside a so-called American “restricted operating zone” just a kilometer (about a half mile) from U.S. troops. He said a bit later a Turkish drone re-entered the restricted area “on a heading toward where U.S. forces were located.”

Commanders determined it was a threat and U.S. F-16 fighter jets shot it down around 11:40 a.m., Ryder said, adding that no U.S. forces were injured.

The incident occurred on the same day as a drone attack killed at least 80 peoplein government-controlled Homs, Syria, where explosive-laden drones were detonated during a military graduation ceremony attended by young officers and their families. An additional 240 people were injured, according to Syria’s health ministry.

Syria’s military blamed insurgents “backed by known international forces,” without naming any particular group, and threatened to respond with “full force.”

Syria has been in a civil war for more than a decade, and the country is split into areas controlled by the Syrian government led by President Bashar al-Assad; al-Qaida-linked militants and Turkish-backed opposition fighters in the northwest; and Kurdish forces in the northeast that the U.S. partners with to conduct missions against the Islamic State group. So far, the war has killed half a million people, wounded hundreds of thousands and left many parts of the country destroyed.

Typically, the U.S. and Turkish militaries, which are NATO allies, work in close coordination in conducting air maneuvers. But Turkey considers the Kurdish forces that work with the American troops to be aligned with the Kurdistan Workers’ Party, or PKK.

The U.S. has about 900 troops in Syria conducting missions to counter Islamic State group militants.

There was no immediate comment from Turkey on the shooting of the drone.