With suspicions on both sides mounting, the United States is struggling to preserve its wobbly partnership with Turkey as it 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 under the surface 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. either supported or was involved in the coup attempt. As the U.S. issued denials, Turkish officials complained loudly that Washington was slow to show support for Turkey's government at its time of greatest need, even though the U.S. expressed support for Erdogan as the coup attempt was underway.

At the same time, the U.S. has been rattled by Turkey's recent diplomatic flirtations with traditional U.S. foes Russia and Iran, concerned they may indicate that a frustrated Turkey is rethinking its allegiance with the West in promoting regional stability. This month Erdogan traveled to Moscow to try to boost ties and possibly even collaboration on ending Syria's civil war, something Moscow has sought unsuccessfully with Washington. And following the Turkish foreign minister's surprise trip to Iran last week, Turkish media reported that Erdogan planned to visit Tehran on Wednesday — the same day he's also slated to meet with Biden.

"Clearly President Erdogan is sending a message by getting closer to Russia and Iran that he's unhappy with the attitude of the West," said Bulent Aliriza, a Turkey analyst at the Washington-based Center for Strategic and International Studies. "Turkey's still going to remain a NATO member and aspire for EU membership, but the atmosphere is worse than it was on July 14," the day before the coup attempt.

A breakdown of ties to Turkey would be problematic for the U.S., which is counting on Turkey to pursue the same approach to fighting the Islamic State group and addressing extremism across the Middle East. Straddling Europe and Asia, Turkey shares borders with Iraq, Iran and Syria, where the porous border has allowed Islamic State fighters in to Turkey, and would-be recruits into Syria.

But the U.S. and Turkey are unlikely to resolve their dispute over Gulen, who has lived in the U.S. for years in self-imposed exile. Gulen has denied any involvement in the coup attempt, in which more than 270 died, but Turkey's government has insisted the U.S. return him to Turkey immediately.

The Obama administration wants more proof before considering extradition. Although Turkey has submitted extradition requests for Gulen, those requests have been based on previous alleged crimes by Gulen and not evidence of involvement in the coup attempt, senior Obama administration officials said.

The U.S. is sending a Justice Department team to Turkey to help sort out the technical requirements of the request, said the officials, who briefed reporters ahead of Biden's trip on condition of anonymity. They added that Biden planned to tell Turkey's leaders that their public allegations of U.S. complicity won't help their cause.

"People have an expectation that Gulen should be returned to Turkey immediately," said Gulnur Aybet, who teaches international relations at Turkey's Bahcesehir University. "If the extradition request is refused or delayed I'm afraid that's going to have serious repercussions."

The U.S. once looked optimistically at Erdogan as a Muslim leader interested in working with the U.S. and pursuing democratic governance. That optimism has been dampened as Erdogan has cracked down on press and other freedoms and his government has put a premium on opposing outlawed Kurdish rebels — sometimes at the expense, in the Obama administration's view — of focusing on IS.

Washington's concerns about human rights and democracy in Turkey intensified after the failed coup, which led the government to detain or fire tens of thousands of police, soldiers, teachers and journalists. Yet the U.S. has been careful not to hammer Turkey over those concerns in public out of concern it would undercut the U.S. message of support for Turkey's government.

The fragile situation in Turkey was compounded over the weekend when a suicide bomber killed at least 54 people at a Kurdish wedding celebration in Gaziantep, near the Syria border. It was the deadliest attack this year in Turkey, joining other deadly attacks by IS or by the Kurdistan Workers' Party, known as the PKK.

In another worrying sign for the U.S., Turkish Prime Minister Binali Yildirim appeared to warm to the possibility of Syrian President Bashar Assad maintaining a role in a transitional government. Turkey and the U.S. have both called for Assad's ouster for years, insisting he can't be part of Syria's future government.

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Suzan Fraser in Ankara, Turkey, video journalist Bram Janssen in Istanbul, and Dan Huff in Washington contributed to this report.

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Reach Josh Lederman on Twitter at http://twitter.com/joshledermanAP