US F-16 warplanes shot down a drone belonging to NATO ally Turkey in Syria on Thursday. While the armed drone did not target US forces in the area, it was deemed too close and a possible threat.

"US commanders assessed that the UAV, which was... less than a half-kilometer from US forces, to be a potential threat and US F-16 fighters subsequently shot down the UAV in self-defense," a Pentagon spokesman told journalists, using the acronym for an unmanned aerial vehicle.

Air Force Brig. Gen. Patrick Ryder called it a "regrettable incident" and said US troops were forced to take shelter in bunkers as Turkey bombed targets nearby.

The decision to shoot down an ally's armed drone "was made out due diligence," Ryder said, adding that "we have no indication that Turkey was intentionally targeting US forces."

US fighting 'Islamic State' in Syria

The United States and Turkey typically coordinate their militaries' air operations in Syria, however, US troops also work closely with Kurdish-led forces to counter the so-called "Islamic State" (IS) terror group in the region.

The US has about 900 troops in Syria to counter IS militants.

A Turkish Defense Ministry official said the drone did not belong to the Turkish armed forces but did not say whose property it was.

Turkey's state-run Anadolu Agency reported that the Turkish intelligence service (MIT) conducted an operation against the Kurdistan Workers' Party (PKK) and its affiliated Kurdish militia group in Syria, known as the People's Defense Units (YPG). The report said Turkey struck suspected arms and ammunition depots and buildings believed to have been used by the groups. It did not provide further details on the operation.

Turkey, the EU and the US all classify the PKK as a terrorist organization.

On Wednesday, Turkish airstrikes hit Kurdish militant targets in Iraq and Syria following a suicide attack outside Turkey's Interior Ministry building in Ankara earlier this week.