European Union leaders hold a summit with Turkey on Sunday, with Ankara set to win a boost for its membership bid and a cash pledge in exchange for its help stemming the flow of migrants.
Turkey's shooting down of a Russian jet on the Syrian border this week and President Recep Tayyip Erdogan's decision to miss the summit will however further complicate an already difficult relationship.
Prime Minister Ahmet Davutoglu, standing in for Erdogan, is expected to secure a deal for the opening in December of a new chapter in Turkey's stalled EU accession talks when he meets its 28 leaders.
The EU is also expected to agree a three-billion-euro ($3.2 billion) aid package for Turkey to help it stop the flow of refugees to Europe from the conflict in Syria, although it will insist on conditions.
EU president Donald Tusk said in his invitation letter to leaders that the summit "will mark an important step in developing our relations and contribute to managing the migration crisis."
Pushed by German Chancellor Angela Merkel to hold the summit as she seeks to ease the refugee burden on her country, Tusk went ahead with the meeting despite a major terror alert in Brussels.
Fuelled by the Syrian war, some 850,000 people have entered the EU this year and more than 3,500 have died or gone missing in what has become Europe's worst refugee crisis since World War II.
Turkey hosts more than two million Syrian refugees and is the main gateway for people trying to get to the EU, via the short sea crossing to the Greek islands.
The EU has pressed Turkey to do more to stop the flow, but Turkey has demanded aid, more summits with European leaders like Sunday's, and the acceleration of its bid for membership of the EU.
Davutoglu pushed Turkey's case ahead of the summit, saying in The Times newspaper that the "terrorist masterminds" behind recent attacks in Paris, Ankara, Beirut and Sinai were trying to "undermine the compassionate treatment of refugees" in the West.
But Brussels and Ankara had different perspectives on the summit, a European source said.
"For the EU the summit is about stemming the migratory flow. For the Turks this is a summit which is about reinvigorating the accession process," the source said on condition of anonymity.
Relations between Europe and Turkey remain tense over Erdogan's increasingly autocratic rule, rights abuses and Turkey's alleged backing for Islamist rebels in Syria.
The EU hit out on Friday at "worrying" spying charges filed against two Turkish journalists.
There are also warnings against relying too much on Turkey, with Hungary's hardline Prime Minister Viktor Orban saying at a migration summit in Malta this month that "we don't want to sit down for talks with Turkey, making them think that they are our last chance of saving us".
The shooting down of a Russian jet by Turkey at a time when the West is looking to build a coalition against the ISIS group "will further complicate an already difficult EU-Turkey dialogue," Ian Lesser of the German Marshall Fund think-tank told AFP.
"Many of Turkey's key demands, including renewed progress on accession talks, and visa liberalisation for Turkish citizens, will be a very hard sell in the prevailing political atmosphere across Europe," Lesser said.
But he said the summit could be the "opportunity for a compromise" with Turkey's hand now possibly weakened by its tensions with Moscow.
Turkish and European officials said the summit would agree to open Chapter 17 of Turkey's accession process -- covering economic and monetary policy -- by mid-December.
The EU leaders will also approve the three-billion-euro aid package, although they will tie it to Turkey's success in tackling the flow of migrants, European officials said.
"At the summit EU states will clearly engage to provide this sum. This does not mean we are writing a cheque to Turkey, but this is money that is linked to projects," a German government source in Berlin said.
Disagreements remain however over the contribution of individual EU member states and whether some can be taken from the EU budget.