WASHINGTON (Reuters) - U.S. Secretary of State Rex Tillerson will not meet members of Turkish opposition groups during a one-day visit to Ankara this week where talks with President Tayyip Erdogan will focus on the war in Syria, senior U.S. officials said on Monday. Thursday’s visit comes at a politically sensitive time in Turkey as the country prepares for a referendum on April 16 that proposes to change the constitution to give Erdogan new powers. A senior State Department official said Tillerson will meet with Erdogan and government ministers involved in the fight against Islamic State in Syria. “It is certainly something we are very acutely aware of and the secretary will be mindful of while he is there,” one State Department official told a conference call with reporters, referring to political sensitivities ahead of the referendum.  American officials expect Erdogan and others to raise the case of U.S.-based cleric Fethullah Gulen, whom the government accuses of orchestrating a failed coup last July. The focus of the Ankara talks is the U.S.-led offensive to retake Raqqa from Islamic State and to stabilize areas in which militants have been forced out, allowing refugees to return home, officials said. A major sticking point between the United States and Turkey is U.S. backing for the Syrian Kurdish YPG militia, which Turkey considers part of the Kurdistan Workers’ Party that has been fighting an insurgency for three decades in Turkey. But the United States has long viewed Kurdish fighters as key to retaking Raqqa alongside Arab fighters in the U.S.-backed Syrian Democratic Forces (SDF). “We are very mindful of Turkey’s concerns and it is something that will continue to be a topic of conversation,” a second U.S. official said. Turkey, Russia and Iran have held Syrian peace talks in Astana, Kazakhstan, on a ceasefire in Syria. The United States has proposed “areas of stability” where Islamic State has been pushed out and refugees can be returned. Six years since the start of the uprising against Syrian President Bashar al-Assad, he is winning on the battlefield although the war is far from over. The once stable country is broken into fiefdoms ruled by rebels and warlords. “We’re looking forward to discussing with Turkey how we can reinforce ceasefire negotiations through the Astana process,” the second U.S. official said. 