US Secretary of State Rex Tillerson's visit to Ankara comes at a politically sensitive time during the run-up to a referendum on April 16 that proposes constitutional changes. These could expand Turkish President Recep Tayyip Erdogan's powers.

During his visit, Tillerson, a former oil executive, will focus on accelerating the battle against the so-called "Islamic State" (IS) in Syria,and Turkey's role in this, experts told DW. 

"To me it seems that this is a preparation for the Raqqa offensive," said Soner Cagaptay, who directs the Turkish Research Program at the Washington Institute for Near East Policy, adding that the US wanted Turkey to be on the supportive side despite its opposition to American assistance to the Kurdish People's Protection Units (YPG).  

"It is one of the reasons why I think Washington has delayed the Raqqa operation until after the referendum (in Turkey), realizing that it makes it more likely for Turkey to be a spoiler if the operation is launched before the referendum," Cagaptay said. He expects the US to assure Turkey that it works with the YPG only in the fight against IS. 

Inclusion of Kurdish fighters 'unacceptable' 

The American support for the Kurdish People's Protection Units (YPG) militia has been a major sticking point between Ankara and Washington. Turkey regards YPG rebels as a "terror group" and an extension of the Kurdistan Workers' Party (PKK), which has waged a decades-long insurgency against the Turkish state.   

Turkey wants a Turkish-backed Syrian rebel force to take the lead instead of the YPG. This diplomatic wrangling has held up the offensive.

Turkey's foreign ministry spokesman Huseyin Muftuoglu reiterated Ankara's stance that Turkish forces were ready to take part in the US-led operations to retake the city of Raqqa from IS. At the same time, he stated that an inclusion of YPG fighters in the US-led operations was "unacceptable". 

Ahead of Rex Tillerson's visit, Turkish Foreign Minister Mevlut Cavusoglu said on Wednesday that Ankara was planning to discuss Syria and the extradition of the US-based Turkish cleric Fethullah Gulen with the most senior US diplomat.

Turkey's role in the Raqqa offensive 

Sinan Ulgen, a former Turkish diplomat and analyst at the Carnegie Europe think tank, expects the differences over the Syrian policy to be on the table during Tillerson's visit in Turkey.

"Ankara has been extremely disturbed over the American support to Syrian Kurds while the Obama administration was in power. It hopes that this policy will change," Ulgen told DW. "Therefore, it has been in touch with the US for the Turkish forces to take part in a Raqqa offensive. But it seems Washington hasn't decided yet on the setup of the operation. However, it looks as if the US will continue with the strategy of the Obama administration."  

A sensitive topic

Ankara's request for the extradition of Gulen is another sensitive topic. Erdogan is accusing the US-based cleric Gulen of having orchestrated the coup attempt in July 2016. However, this problem concerns the US Department of Justice, underlined Cagaptay: "That's out of question. I don't think the Secretary of State can promise anything on an issue that has been referred to the courts. That doesn't work in the American system."

On Thursday, Tillerson will meet Turkish President Recep Tayyip Erdogan and several government ministers, but no opposition groups, Reuters reported on Monday, citing senior US officials.