American troops in Syria may be on the receiving end of an “Ottoman slap” if they don’t abandon a key group of local fighters, Turkish President Recep Tayyip Erdogan threatened Tuesday.

“It is clear that those who say ‘we will respond aggressively if you hit us’ have never experienced an Ottoman slap,” Erdogan told his parliament, per local media.

The Ottoman Empire, based in modern-day Turkey, dominated much of the Middle East and southeastern Europe for more than 600 years. It was broken up by the Allied powers following the First World War. Erdogan's boast was a response to an Army general who warned pro-Turkish forces not to attack a Syrian city held by U.S. and counter-Islamic State coalition forces that the Turks regard as a terrorist threat in their own right. Syrian militias partnered with Turkey wanted to overrun Manbij, where U.S. military advisors train Kurdish and Arab fighters.

“You hit us, we will respond aggressively,” Lt. Gen. Paul Funk told the pro-Turkish militias, according to the New York Times. “We will defend ourselves.”

Erdogan’s sharp rhetoric comes just days before Secretary of State Rex Tillerson arrives in Ankara for meetings that the U.S. hopes might defuse tensions. But Turkey is in the midst of a campaign near a city called Afrin against Syrian Kurds, whom they regard as partners of a Turkish Kurdish terrorist group known as the PKK. On Tuesday, Turkey also announced a series of airstrikes on what it called PKK positions in Kurdish Iraq.

“As to the situation in Afrin, it has detracted from our fight to defeat ISIS in eastern Syria,” Tillerson told reporters in Kuwait. “So, I will be traveling to Ankara later in the week. We’ll have further discussions with them about how do we continue to work together on the essential mission, which is to defeat ISIS, recognizing, as we all know, there are other terrorist elements operating inside of Syria, including al Qaeda, which is a threat to the United States, Nusrah Front, and others.”

Turkey wants the U.S. to stop arming the Syrian Kurds and take back the weapons previously provided to them during the counter-ISIS fight. “Ties with the U.S. are at a very critical point,” Foreign Minister Mevlüt Çavuşoğlu told reporters in remarks published overnight. “We will either fix these relations or they will break completely.”