Brussels/Istanbul – Turkey applied for EU membership in 1987. Relations between the two sides became increasingly strained in recent years, hitting a low following a coup attempt in 2016 and an ensuing crackdown against critics of President Recep Tayyip Erdogan, including a wide-ranging clampdown on the opposition and media.
Top EU officials are due to meet Erdogan in the Bulgarian resort of Varna. Here are some of the issues that could surface:
Turkish EU membership
Progress in Turkey's EU membership bid has always been slow, in part due to reluctance from France and Germany and issues over Ankara's stance on human rights.
Negotiations between Brussels and Ankara have ground to a halt since the 2016 foiled coup. EU member states have stopped short of a formal suspension – even as the European Parliament called for such a move – meaning that Turkey still receives financial support for efforts to meet EU standards.
Turkey is still pushing to join the European Union but Turkish Foreign Minister Mevlut Cavusoglu recently said: "We have no expectations from the EU regarding membership."
EU officials often point to a migration deal struck with Turkey in early 2016 as an example that the two sides can work together. Under the deal, Ankara has stemmed a flow of refugees heading for Europe – many of them fleeing the war in Syria.
In return, the EU has pledged an overall 6 billion euros (7.4 billion dollars) in aid. Europe says it has contracted out the first batch of 3 billion, although Turkey argues that it has received only 850 million euros.
Ankara wants the cash directly, whereas Brussels insists on funding projects that support refugees in Turkey. A key charity that worked with European funds was shut down as part of a government crackdown on civil society organizations.
As part of the migration deal, the EU promised to lift visa requirements for Turkish citizens visiting the bloc, once certain preconditions have been met.
The two sides have since been arguing over the issue, with Ankara threatening to cancel the refugee deal over the EU's failure to lift visa requirements, while Brussels says not all conditions have been met. Turkey's vague anti-terrorism laws are seen the main problem, as the EU says they are used against political opponents and violate rights.
The EU has been critical of Turkey's crackdown on judges, media and opposition voices following the failed 2016 coup. Tens of thousands are in jail, including some 150 journalists and media workers, while tens of thousands more were fired from the civil service.
In January, European Commission President Jean-Claude Juncker said the release of journalists was necessary before relations with Turkey could improve. Judges in Turkey recently released some prominent journalists, although others received life sentences.
The EU has called on Turkey – along with Russia and Iran – to play its role in reducing violence in Syria, after Turkey began a military operation in the Kurdish enclave of Afrin. The European Parliament has gone further, calling on Ankara to to withdraw its troops from Syria. Turkey has scoffed at the demand. Ankara views the Kurdish forces in Syria as a terrorist threat with links to the banned rebel Kurdistan Workers Party (PKK).
Turkey has called on EU member states to take a firmer stance against supporters of the Kurdish nationalist movement, arguing that Europe is harbouring potential terrorists linked to the PKK.
Ankara also wants EU member states to send back at least a share of Turkish asylum seekers who are Kurdish or who support the movement of US-based cleric Fethullah Gulen, who Ankara says orchestrated the 2016 coup attempt.
EU member Greece has demanded the return of two soldiers who were arrested and charged with military espionage after wandering across the border into Turkey during a patrol on March 1.
Ankara, meanwhile, has been pressing for the extradition of eight soldiers who defected to Greece following the failed 2016 coup and nine other nationals arrested in possession of weapons in Athens in November, ahead of a visit by Erdogan. Greece fears that both groups may not receive a fair trial.
Turkey recently prompted a diplomatic stand-off when it blocked the exploration of a potential natural gas field off the coast Cyprus by positioning warships near a drilling rig owned by Italian energy company ENI.
The island of Cyprus has been divided since 1974 into a predominantly Greek Cypriot south, which is an EU member, and a Turkish Cypriot north whose sovereignty is only recognized by Ankara.
The EU has been critical of Turkey's intervention against the rig, while Ankara has warned against the "unilateral" moves by Cyprus to explore and exploit natural resources.
Since 1995, Turkey has been a member of the EU's customs union, allowing for the free circulation of industrial goods. Ankara would like this to be expanded, to include more agricultural goods, services and public procurement. The EU has said it is open to this, but Turkey would have to first respond to EU concerns over the rule of law.