The White House envoy coordinating the US-led coalition fighting Daesh (ISIS) was holding talks in Ankara Friday a day after visiting a Syrian Kurdish militia deemed by Turkey to be "terrorists".
The visit by Brett McGurk, which was not announced in advance, comes a month after Turkish Foreign Minister Mevlut Cavusoglu called for McGurk to be fired, claiming he was supporting Kurdish militants.
It also comes as US-backed Kurdish-dominated forces close in on the Syrian city of Raqa, the self-declared capital of Daesh.
McGurk was to meet with Cavusoglu's deputy undersecretary, Sedat Onal, foreign ministry sources said.
Turkish media reported that he was also to meet with defence ministry officials, but no details were given.
Tensions between Washington and Ankara have escalated over US support for the Kurdish People's Protection Units (YPG) in Syria, which Turkey considers a front for outlawed Kurdistan Workers' Party (PKK) separatists.
The US, Turkey and the European Union have labelled the PKK a "terrorist group".
Washington has provided weapons to YPG fighters in the Syrian Democratic Forces (SDF), despite warnings from Turkey. The forces have trapped Daesh fighters in Raqa after cutting off a last escape route Thursday.
McGurk was in the northern Syrian town of Tabqa on Thursday, which the SDF captured from Daesh on May 10. He met with the Tabqa Civil Council, which is administering the town's day-to-day affairs.
The US has sought to assuage Turkey's concerns about arming the Kurdish forces, with US Defence Secretary Jim Mattis promising his Turkish counterpart Fikri Isik last week to take back weapons from the YPG once the jihadists are defeated.
The YPG controls about 20 percent of Syrian territory but roughly three-quarters of the northern border with Turkey.
President Recep Tayyip Erdogan has repeatedly said Ankara will oppose what it terms a "terrorist corridor" on its southern border established by the YPG.
Last August, Ankara launched its Euphrates Shield offensive aimed at ousting Kurdish militias and Daesh fighters from the border area.
Although the operation ended in March, Turkish officials have indicated there could be another offensive in Syria if necessary.
Local media reports this week suggested a military buildup on the Turkish border close to the Kurdish-held Afrin region, but there has been no official confirmation
