Turkey says are a branch of a domestic Kurdish militant organization, but USA officials.

"I think the longer term status of President Assad will be decided by the Syrian people", Tillerson told a joint press conference with Turkish Foreign Minister Mevlut Cavusoglu in Ankara on Thursday.

Turkey "has suffered more losses to terrorism than all the Europeans combined", according to a senior State Department official. Turkish leaders feared the YPG, aided by the United States, would push west of the Euphrates River and link up with several other Kurdish tribes to form a semi-autonomous group similar to the province in northern Iraq that largely operates independent of the central government in Baghdad.

"These are not easy decisions", he said, referring to the debates with Turkey over the battle plans for Syria.

"They are hard options, let me be very frank".

And despite Trump and Erdogan's talk of improving U.S. Turkey and the United States disagree sharply on how to combat Islamic State: Washington supports Kurdish militias that Erdogan regards as an arm of the Kurdistan Workers' Party (PKK), which Turkey and the US consider a terrorist organization. Turkey views them as an extension of the PKK, a militia group that has fought a three-decade insurgency in Turkey.

The two sides are also at loggerheads over Fethullah Gulen. -Turkish ties, the countries continue to clash over many matters, including the fate of a Pennsylvania-based cleric that Turkey blames for a failed coup attempt last July. American officials have said the evidence provided by Turkey so far is insufficient to make a legal case for extradition. The visit follows months of political upheaval and a series of devastating terrorist attacks that have left Turkey reeling - and at times at odds with the U.S.

Something may have changed this week, however.

Asked whether the USA government would change its stance on the YPG/PKK, Tillerson again did not answer, saying only that discussions continued with all coalition members, including Turkey, over how to find solutions for Syria. It then moved south to al-Bab, an ISIS stronghold where Yildirim said "everything is under control".

But the United States embassy in Turkey said its Istanbul consulate contacted theology lecturer Adil Oksuz only to inform him that his American visa had been revoked. Elizabeth Palmer reports from Ankara.

"There will be little surprise if USA sides with the PYD over Raqqa".

Prime Minister Binali Yildirim on March 29 said the operation was "successful and is finished". "How do we destroy ISIS, while maintaining a relationship with a critical North Atlantic Treaty Organisation ally?" "Everyone can see it's happening", observes former senior Turkish diplomat Aydin Selcen, who served across the region and in Washington.

Shortly after Tillerson's arrival in Ankara, Justice Minister Bekir Bozdag told broadcaster A Haber that the arrest was a "completely political move" created to tarnish Turkey and Erdogan, and questioned the evidence in the case.

Erin Cunningham contributed from Istanbul.