Russia started delivering advanced missile defense equipment to NATO member Turkey on Friday, the Defence Ministry in Ankara said, setting the stage for likely U.S. sanctions on Ankara. The dispute over the S-400 air defense missiles, which the United States says are incompatible with NATO military systems and could threaten U.S. F-35 stealth fighter jets which Turkey has also ordered, is one of several issues which have frayed ties between the two allies. Pictured: A view shows a new S-400 "Triumph" surface-to-air missile system after its deployment at a military base outside the town of Gvardeysk near Kaliningrad, Russia March 11, 2019. REUTERS/Vitaly Nevar 

SYRIA TENSIONS:

Turkey is furious about U.S. support in Syria for the Kurdish YPG militia, which Ankara sees as a terrorist group. Washington is coordinating with Ankara and the YPG to establish a safe zone on Turkey's southern border. Ankara wants YPG fighters to withdraw from the area to secure its border, and Washington wants guarantees that the U.S.-backed YPG forces which defeated Islamic State will not be harmed. Pictured: Kurdish fighters from the People's Protection Units (YPG) fire rifles at a drone operated by Islamic State militants in Raqqa, Syria, June 16, 2017. REUTERS/Goran Tomasevic 

U.S. CONSULAR DETAINEES:

A Turkish court in June refused to release Metin Topuz, a translator and fixer for the U.S. consulate in Istanbul, who will be held at least until his next appearance on Sept 18.

His arrest in October 2017 led to a diplomatic dispute and suspension of visa services by both countries. Topuz is one of three U.S. consulate employees who have been charged in criminal cases that have been major irritants relations with Washington. Pictured: U.S. Consulate in Istanbul, Turkey, October 11, 2017. REUTERS/Murad Sezer 

U.S. MIDDLE EAST POLICY:

Erdogan has bitterly criticized U.S. support for Israel. He said its decision to recognize Jerusalem as Israel's capital meant Washington had forfeited its role as a mediator in the region, while Trump's support for Israeli sovereignty over the Golan Heights, captured from Syria in a 1967 war, could trigger a crisis. Pictured: Senior White House Adviser Ivanka Trump and U.S. Treasury Secretary Steven Mnuchin stand next to the dedication plaque at the new U.S. embassy during the dedication ceremony in Jerusalem, May 14, 2018. REUTERS/Ronen Zvulun 

VENEZUELA:

When Trump gave his public backing to Venezuelan opposition leader Juan Guaido earlier this year, Erdogan said he was shocked by the U.S. president's move, and responded by calling President Nicolas Maduro to tell him: "Stand tall." Pictured: Venezuelan opposition leader Juan Guaido gestures from a statue of Venezuela's independence hero General Francisco de Miranda, at a rally against the government of President Nicolas Maduro and to commemorate the 208th anniversary of the country's independence in Caracas, Venezuela July 5, 2019. REUTERS/Carlos Garcia Rawlins 

HALKBANK:

Last year a U.S. court sentenced Mehmet Hakan Atilla, a Turkish citizen and banker at Turkey's state-controlled Halkbank, to 32 months in prison after he was convicted of taking part in a scheme to help Iran evade U.S. sanctions.

Washington is also considering a fine against Halkbank.

Federal prosecutors accused Atilla of conspiring with gold trader Reza Zarrab and others to elude U.S. sanctions using fraudulent gold and food transactions. Zarrab pleaded guilty and testified for the prosecutors.

Zarrab described a scheme that he said included bribes to Turkish government officials and that was carried out with the blessing of Erdogan. Erdogan has condemned the case as a political attack on his government.

Attila is due to be released on July 19. Pictured: A street vendor sells roasted chestnuts in front of a branch of Halkbank in central Istanbul, Turkey, January 10, 2018. REUTERS/Murad Sezer 

TURKEY'S FAILED COUP AND EXILED CLERIC

Turkey demands that the United States extradite Islamic cleric Fethullah Gulen, who Ankara has said orchestrated a failed 2016 military coup against Erdogan.

U.S. officials have said the courts would require sufficient evidence to extradite the elderly Gulen, who has denied any involvement in the coup. Pictured: U.S.-based Turkish cleric Fethullah Gulen at his home in Saylorsburg, Pennsylvania, July 10, 2017. REUTERS/Charles Mostoller