European Commission chief Jean-Claude Juncker said he believed the 28-member bloc could seal a deal with Turkey on Thursday that would help curb the continent's migrant crisis.

"I am cautiously optimistic that we will conclude a binding agreement with Turkey," Juncker said on Twitter, reiterating comments he made to the German financial paper "Handelsblatt."

The EU is hoping for a deal that will see Turkey prevent refugees from making dangerous sea journeys across the Aegean, and take back thousands of illegal migrants who are currently stuck in Greece.

Nowhere to go

More than 143,000 people have arrived at the EU's southern borders since January 1 alone. But coordinated border closures by Balkan countries have left tens of thousands stranded in temporary camps in northern Greece.

Under an idea mooted by Turkish Prime Minister Ahmet Davutoglu, for each Syrian migrant Ankara accepts, it would send one to the EU in a more orderly redistribution program.

The draft agreement, seen by the Reuters new agency, says the main objective is "to break the business model of [people] smugglers and to offer migrants an alternative to putting their lives at risk."

Is the plan legal?

The draft stresses the proposals are "a temporary and extraordinary measure which is necessary to end the human suffering and restore public order." But some experts have questioned whether sending asylum-seekers back en masse to Turkey would breach international law.

On Wednesday, German Chancellor Angela Merkel threw her weight behind the deal, telling the Berlin parliament the proposals offer "a real chance" of a solution, while vowing the EU would stand firm on human rights.

She also paid tribute to Ankara's response to the Syrian refugee crisis, saying "What Turkey has done for... some 2.7 million refugees can't be praised highly enough."

Mistrust of Turkey

But the initiative still faced fire on several fronts, including from Germany's Left Party leader Dietmar Bartsch, who denounced negotiations with Turkish President Recep Tayyip Erdogan.

"You're trying to win points with a man who has turned Turkey into a player in the Syrian war and his country into a transit zone for terrorists," he told parliament.

France has warned against attempts by Turkey to strong-arm Europe, and Czech President Milos Zeman charged that Ankara's request for billions of euros more in EU aid amounted to "blackmail."

Brussels already pledged 3 billion euros ($3.36 billion) to Turkey last year to keep migrants from traveling onward to Europe. A second sum of 3 billion euros is now on the table.

EU leaders are set to discuss the scheme on Thursday before they are joined by Turkish PM Davutoglu on Friday morning.

Carrot dangled

In return for Turkey's cooperation, the EU is likely to offer to open new "chapters" in negotiations for Turkey to join the European Union, which could be preceded by a liberalization of trade and a relaxation of EU entry requirements for Turkish citizens.

But observers say a key stumbling block has been set up by Cyprus, which threatens to veto the plan over its territorial feud with Turkey.

The eastern Mediterranean island has been divided since 1974, when Turkish troops invaded its north in response to a Greek-engineered coup attempt.

Juncker met with Cypriot President Nicos Anastasiades late Wednesday to discuss the issue, after EU Council President Donald Tusk jetted to Nicosia on Tuesday for emergency talks.

Tusk, who will chair the summit in Brussels on Thursday and Friday, was less optimistic than Juncker, saying much still needed to be done to reach a deal with Turkey.