Turkey on Saturday called on U.S. officials to detain an Islamic leader based in the Poconos, whom they blame for last month's abortive Turkish military coup attempt that killed more than 200 people.

The Chief Prosecutor's Office in Istanbul, Turkey's largest city, wrote the letter demanding the U.S. detain Fethullah Gulen while they prepare a formal extradition request with proof of his wrongdoing.

In the letter, prosecutors call Gulen the leader of "Fethullah Terrorist Organization" and blame his group for leading the coup attempt, the Turkish national Anadolu news agency said in a statement. Turkey has said it has fired nearly 5,000 soldiers and public employees, and suspended more than 75,000 government employees, in the aftermath.

Gulen has repeatedly denied any involvement and denounced the coup attempt as a "catastrophic tragedy" that violates the values of his Hizmet (Service) movement. He has asked for an international inquiry into the coup attempt, promising to accept punishment if found responsible.

Secretary of State John Kerry has said the U.S. needs proof of Gulen's involvement before it will extradite him.

A U.S. failure to hand over Gulen "will inevitably affect the bilateral relations of the two countries," Turkey's foreign minister, Mevlut Cavusoglu, said in a separate statement published by Anadolu.

The letter to U.S. officials says Gulen faces charges including "attempting to eliminate the government of Republic of Turkey," "attempting to assassinate the president," and "attempting to demolish the Turkish parliament."

The demand comes as Turks await the arrival of Vice President Biden, who plans to visit Aug. 24.

"The U.S. stance is getting better," Prime Minister Binali Yildirim told reporters Saturday, according to the independent Turkish newspaper Hurriyet. "The U.S. cannot ignore our demand. It is crystal clear that Fethullah Terrorist Organization is behind the failed coup."

Gulen, 75, describes himself as a pacifist opposed to war and violence, an Islamic mystic in Turkey's Sufi spiritual tradition, and a supporter of Western-style education and democracy.

His Hizmet movement maintains a network of schools and charities in Western, Islamic, and African nations, including taxpayer-funded U.S. charter schools. He lives at a 26-acre retreat in rural Saylorsburg, about 90 miles north of Philadelphia.

The Alliance for Shared Values, a New York-based nonprofit group that is part of Gulen's movement, issued a statement Saturday defending Gulen.

"As we have said repeatedly, there is no reason to detain Mr. Gulen. He has consistently denounced the coup and denied his involvement," the nonprofit said in a statement. "Despite what Turkish government officials and what government-controlled Turkish media say, the United States has a process based on evidence to consider such requests from foreign countries. We are still waiting for the Turkish government to present any credible evidence."

Turkey has the second-largest military force in NATO, the North Atlantic alliance set up by the U.S. to oppose the Soviet Union and other regional threats since World War II. The U.S. operates key air bases at Izmir and Incirlik.

Though allies, the U.S. and Turkey support rival factions in the Syrian civil war, which has sent more than a million refugees fleeing to Turkey.

The U.S. has armed - and has sent jet bombers and special forces soldiers to support - secular Kurdish militia who, with local allies, have recaptured towns from the Islamic State, which has promoted terrorist attacks against the U.S. and other countries.

The attempted coup, the Syrian civil war, and the conflict between Turkey's military and minority Kurdish organizations have tested the U.S.-Turkey alliance.

Turkey shot down a Russian fighter jet that was attacking Turkish allies in Syria last year. That led to a freeze on commercial relations between the two countries. But President Recep Tayyip Erdogan visited Russian leader Vladimir Putin last week, putting the U.S. on notice it can't take the alliance for granted.

Erdogan last week called Putin his "dear friend." NATO then called Turkey a "valued ally."

U.S. officials and other Western leaders have said they are worried Erdogan has used the coup attempt to step up arrests and firings of journalists, college professors, and other critics of his ruling party; Turkey says Western leaders have failed to offer support at a difficult time.

In Syria, Turkey has objected to the U.S.-Kurdish alliance, complaining the Syrian Kurds are allied to Kurdish separatists in Turkey, who have killed Turkish police, military, and civilians in gun and bomb attacks.

Gulen and his Turkish supporters were formerly influential allies of Erdogan, who has been working to restore a larger role for Islam in Turkey and has supported Islamist groups in Syria and Egypt.

But Erdogan and Gulen are now enemies and have accused each other of seeking dictatorial powers.

Since the July 17 coup attempt, which was defeated by police, military units, and citizens of Istanbul and Ankara, Turkey's capital, Erdogan has been purging Gulen supporters from Turkish schools, the military, and the government. In statements on his website, Gulen has said he faces "harassment" in Turkey, and hopes to stay in Pennsylvania to ensure his "tranquillity."

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Jeremy Roebuck contributed to this article.