Washington has said Turkey's operation against the YPG militia in Syria risks diverting energy from the fight against IS jihadists
US Secretary of State Rex Tillerson arrived in Turkey on Thursday for talks with President Recep Tayyip Erdogan, seeking to ease tensions between the NATO allies that reached new heights over Ankara's ongoing operation inside Syria.
A prime task of President Donald Trump's top diplomat will be to allay Turkish anger over US policy in Syria, a dispute which has ignited the biggest crisis in bilateral ties since the 2003 Iraq war.
The former chief of energy giant Exxon Mobil, who is on a multi-leg tour of the Middle East, headed to talks with Erdogan at his presidential palace after landing in Ankara from Beirut.
Tillerson will meet Foreign Minister Mevlut Cavusoglu on Friday morning.
Turkey's operation against the People's Protection Units (YPG) Kurdish militia in the Afrin region of Syria has added a potentially insurmountable new problem to an increasingly rough bilateral relationship.
Analysts said the level of tension was similar to 2003 when Turkey refused to let US troops operate from its territory for the Iraq war, or even the aftermath of Ankara's invasion of Cyprus in 1974.
Turkey's operation against the YPG, which Ankara blacklists as a terror group, has seen Turkish troops fighting a militia which is closely allied with the US in the battle against Islamic State (IS) jihadists.
Speaking in Beirut hours before his meeting with Erdogan, Tillerson denied Washington ever gave heavy weaponry to the YPG and thus could not gather up such arms, as desired by Ankara.
"We have never given heavy arms to the YPG so there is none to take back," Tillerson said.
- Weakening anti-IS efforts -
Erdogan earlier this month accused Washington of sending in thousands of truckloads and planeloads of weapons to the YPG in Syria, asking why the US still had a presence there if the jihadists had been defeated.
And Erdogan has further upped the ante by warning US troops to leave Manbij, a YPG-held town east of Afrin, raising fears of a clash between the allies.
He also warned that the US risked being dealt an "Ottoman slap" in Syria -- a backhand thwack which, according to legend, could kill an opponent at a stroke.
For Ankara, the YPG is a branch of the outlawed Kurdistan Workers' Party (PKK), which is blacklisted as a terror outfit by the US and the EU.
But for Washington, the YPG is an ally against IS jihadists and Turkey's operation is a distraction from efforts to ensure the Islamist extremists are permanently defeated.
Speaking ahead of the visit, a senior State Department official said "eyes had to be on" the defeat of IS.
"It's complicated enough. Let's not make it more so."
But Cavusoglu this week warned Washington that ties were at a "critical point" at which relations would "be fixed or... completely damaged."
- Litany of problems -
The squabble over Syria is, however, just one of a litany of issues burdening Turkey-US relations.
Ties were damaged after the failed coup of 2016 with Turkey stung by a perceived lack of US solidarity and angered by Washington's refusal to extradite Fethullah Gulen, a Pennsylvania-based cleric blamed for ordering the putsch.
There is still no US ambassador to Turkey after the departure of John Bass last year, and it was only in December that the two sides ended a row following tit-for-tat suspensions of visa services.
Last month, Ankara reacted furiously to the conviction in New York of Turkish banker Mehmet Hakan Atilla on charges of violating sanctions against Iran.
And Washington has expressed concern that several of its citizens -- as well as at least two Turkish employees of US missions -- have been caught up in the post-coup crackdown.
Turkish President Recep Tayyip has warned Washington it risks getting an "Ottoman slap" if it interferes with its Syrian operation
Last week, NASA scientist Serkan Golge, a dual national, was jailed for seven-and-a-half years for being a member of Gulen's movement, with the State Department saying he had been convicted "without credible evidence".
Another case is that of US pastor Andrew Brunson, who ran a church in Izmir, who has been held on similar charges since October 2016.
Such tensions have hit Turkish public opinion, with 83 percent holding unfavourable views of the US, a Center for American Progress poll showed this week.