WASHINGTON — With suspicions on both sides mounting, the United States is struggling to preserve its wobbly partnership with Turkey as the NATO ally entertains a closer relationship with Russia and fumes over a U.S.-based cleric it blames for orchestrating last month’s failed coup attempt.

Vice President Joe Biden faces a difficult mission when he travels to Ankara on Wednesday to try to smooth over recent strains: He comes bearing no assurances that the U.S. will agree to Turkey’s demand that it extradite that cleric — Fethullah Gulen, who lives in Pennsylvania. Instead, he’ll try to convey that the U.S. still needs and values Turkey as a key NATO ally, even amid worrying signs that the U.S. and Turkish approaches to the region’s conflicts may be diverging — especially on Syria.

Tensions between the two countries were already bubbling before the attempted overthrow on July 15, but have since burst into the open. U.S. leaders were incensed when Turkish President Recep Tayyip Erdogan alleged the U.S. was involved in the coup attempt.