Turkey-US: main points of discord

US Secretary of State Rex Tillerson arrives in Turkey on Thursday with several issues souring the relationship between the two NATO allies.

Here is a rundown of the main sticking points.

Syria, the Kurds

Turkey in 2015 agreed to allow the US-led coalition fighting the Islamic State group in Syria to use a key air base and has been part of the coalition against the jihadists.

But its opposition to Syrian Kurdish militia, who dominate the Syrian Democratic Forces (SDF) supported by the coalition, has complicated its role.

The Kurdish People's Protection Units (YPG) militia is an ally of the United States but Ankara accuses it of being a terror organisation.

Ankara says the YPG is the Syrian offshoot of its outlawed Kurdistan Workers' Party (PKK), which has waged a three-decade rebellion in Turkey's southeast.

The YPG controls key Syrian northern towns and Turkey fears the emergence of an autonomous Kurdish region in Syria, which could have implications for its own territory.

In 2016 Turkish troops launched an operation in Syria to drive out Islamic State fighters as well as Kurdish militias.

But Turkey was angered when the US-led coalition said in January it was working to create a 30,000-strong border security force, around half of whom would be retrained members of the SDF.

It launched a new air and ground operation to oust Kurdish militia in northwest Syria, defying US warnings it risked destabilising the area.

US-based Turkish cleric

In July 2016 a renegade army faction attempted a coup against President Recep Tayyip Erdogan which left 249 people dead, not including the plotters.

Erdogan, on vacation at the time, blamed the coup attempt on Muslim preacher Fethullah Gulen, a former ally who lives in Pennsylvania.

Erdogan has demanded the United States extradite the cleric but to no avail.

Turkey has since carried out major purges of the civil service to remove Gulen supporters.

In 2017 a Turkish employee of the American consulate in Istanbul was arrested on suspicion of links to Gulen. A Turkish staffer at the US consulate in Adana was detained on suspicion of links to the PKK.

Washington rubbished the charges and in response suspended the majority of its visa services for Turkey. Ankara did the same. The restrictions were lifted at the end of 2017.

Media, detentions

In 2016 former US president Barack Obama warned that Turkey's approach towards the media was taking it "down a path that would be very troubling."

According to the P24 press freedom group, there are 156 journalists behind bars in Turkey, most of whom were arrested after the failed coup bid.

The United States also criticised the detaining in 2016 of several US citizens for allegedly being part of the Gulen network.

NASA scientist Serkan Golge, a dual national, was jailed for seven-and-a-half years last week for being a member of Gulen's movement, with the State Department saying he had been convicted "without credible evidence".

US pastor Andrew Brunson, who ran a church in Izmir, has been held on similar charges since October 2016.

Halkbank trial

In January Ankara reacted furiously to the conviction in New York of the deputy chief executive of Turkish lender Halkbank, Mehmet Hakan Atilla, on charges of violating sanctions against Iran.

The trial implicated former Turkish ministers and even Erdogan in the sanctions-busting scheme.

Erdogan said it was part of a "chain of plots" against Turkey. Atilla is due to be sentenced on April 11 and Halkbank risks a massive fine.

Several of Erdogan's bodyguards have meanwhile been indicted in absentia over a fracas during his visit to the United States last year.