US-backed Kurdish militants in Syria are merely an extension of PKK separatists, who are labeled a terrorist organization by the US, European Union and the North Atlantic Treaty Organization, Erdogan said on Friday in Istanbul.

The Turkish leader was hitting back at a US statement on the extension of “national emergency” with regards to Syria, where President Joe Biden said Turkish military operations were threatening peace and stability. The US stance is out of line with the spirit of NATO, Erdogan said.

“Activities carried out by the US with Syrian affiliates of PKK in that country constitute an extraordinary threat for Turkey’s national security,” the Turkish leader said.

Turkish opposition against American support for Kurdish militants in Syria is hardly new. But Erdogan has stepped up his criticism of the US over the past week, after US jets shot down a Turkish combat drone over Syria. The drone was operating as part of cross-border strikes Turkey launched following a suicide bombing targeting its capital city, an attack Erdogan said was carried out by Kurdish militants that came from Syria.

“There’s a security issue between the US and Turkey,” Erdogan said. “Aren’t we together with the US in NATO? Did the US shoot our drone? Yes, it did. How are we together in NATO? How can you do this?”