Washington DC - The US military has shot down a Turkish drone in Syria after it came close to US forces, the Pentagon said on Thursday.

"This is certainly a regrettable incident," Pentagon spokesperson Pat Ryder said in Washington.

The drone was approaching US troops when US commanders on the ground decided that there was a "potential threat" and, as a result, a US F-16 fighter jet shot down the drone, the spokesperson said.

Ryder stressed that there was currently no evidence that Turkey had deliberately targeted the US military. No US forces were injured, he said.

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Earlier on Thursday, the Turkish military launched an air-ground offensive in northern Syria, in response to an attack in the capital Ankara carried out by the banned Kurdistan Workers' Party (PKK), according to media reports. The PKK is listed as a terrorist organization in Turkey, Europe, and the US.

The Syrian Democratic Forces (SDF), led by the Kurdish militia YPG, however are a close ally of the US in the fight against Islamist terrorists in Syria. In its latest offensive, Turkey is reportedly primarily targeting the SDF, which it also considers a terrorist organization.

US Defense Secretary Lloyd Austin discussed the incident with his Turkish counterpart, the Pentagon spokesperson said. "We will never question Turkey's legitimate right to protect its people from terrorists," Ryder said, adding that the US stands "firmly with our NATO ally Turkey."

"We do remain concerned about the potential impacts of military escalation in that region," he added.

The US has approximately 900 troops deployed in Syria as part of international efforts to combat the Islamic State (IS) group, and carries out frequent raids targeting the jihadists.