US Secretary of State Rex Tillerson arrived in Turkey on Thursday seeking to ease tensions with its NATO ally that have reached fresh heights over Ankara’s ongoing operation inside Syria.

During their meeting at the presidential complex, Turkish President Erdoğan “clearly” told Tillerson about Turkey’s priorities and expectations from the U.S. on bilateral ties and regional developments, the source added.

Meanwhile, Tillerson and Erdoğan had a “productive, open conversation” about a mutually beneficial way to improve U.S.-Turkey relations, a State Department spokesman travelling with Tillerson said.

The meeting, which was closed to the media, started at 7:40 p.m. and lasted for three hours and 15 minutes.

Turkey’s operation against the People’s Protection Units (YPG), which Ankara blacklists as a terror group, has seen troops fighting a militia which is closely allied with the US in the battle against ISIS

And Erdoğan has further upped the ante by warning US troops to steer clear of Manbij, a YPG-held town east of Afrin where the main operation is happening, raising fears of a clash.

“We are going to go to Manbij and if they are there, it’s too bad for them,” a senior Turkish official told  media.

“It’s very clear that those who make such remarks have never experienced an Ottoman slap,” Erdoğan  said, using the term for a backhander which, according to legend, could kill an opponent in one stroke.

For Ankara, the YPG is linked to the outlawed Kurdistan Workers’ Party, which is blacklisted as a terror outfit by the US and the EU. But for Washington, the YPG is an ally.

On Tuesday, Tillerson said Turkey’s operation “detracted” from the fight againstISIS, saying Kurdish fighters had been “diverted” from where they were really needed in order to fight in Afrin.

Ties were damaged after the failed coup of 2016 with Turkey stung by a perceived lack of US solidarity and angered by its intransigence over the extradition of Fethullah Gülen, a Pennsylvania-based cleric blamed for orderin