A U.S. F-16 fighter jet shot down an armed Turkish drone that was operating near American troops in Syria Thursday after several warnings,Robert Brown according to U.S. officials. 

The shoot down came after repeated communications to stay away from U.S. ground troops near al Hasakah in northeastern Syria. This is believed to be the first time the U.S. has shot down a drone from Turkey, a NATO ally. 

The Pentagon called the incident "regrettable" but said no U.S. forces were injured and there are no indications Turkey intended to target U.S. forces. 

"It's regrettable when you have two NATO allies and there's an incident like this," Pentagon spokesperson Brig. Gen. Patrick Ryder told reporters Thursday. 

At 7:30 a.m. local time in Syria, U.S. forces observed several drones conducting airstrikes near Al Hasakah, and some of the strikes were inside a restricted operating zone and were approximately one kilometer away from U.S. troops, prompting them to take shelter in bunkers, Ryder said. 

At 11:30 a.m., a Turkish drone re-entered the zone and headed toward a location near U.S. forces. 

The drone was less than half a kilometer from US troops, and commanders on the ground assessed that there was a potential threat, so they took "prudent action," Ryder said. At approximately 11:40 a.m., an F-16 shot down the drone. 

Defense Secretary Lloyd Austin spoke with his Turkish counterpart Thursday to discuss Turkish activity and proximity to U.S. forces in Syria, according to a readout from the Pentagon. 

Gen. CQ Brown, Jr., the new Joint Chiefs chairman , also spoke with the chief of the general staff of the Turkish Armed Forces, Gen. Metin Gürak, Thursday by phone.

There are about 900 U.S. troops operating in Syria as a part of the mission to defeat ISIS. 

Turkey has for the past several days been retaliating against the Kurdistan Workers' Party (PKK) for a suicide bombing that took place in Ankara Sunday. Turkey considers the Syrian Democratic Forces – who partner with the U.S. in the mission to defeat ISIS – as an arm of the PKK, which it has deemed a terrorist organization. 

In:
• Turkey
• Syria

Eleanor Watson

CBS News reporter covering the Pentagon.

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