European Union leaders begin a difficult summit on Thursday to push for a crucial agreement with Turkey to curb the continent's massive migration crisis despite threats by Cyprus to quash the deal.

The proposed plan, which would make Turkey a central pillar of the divided EU's response to the crisis, has been hailed by German Chancellor Angela Merkel as the "first real chance" to end the influx of refugees and migrants.

But the ambitious accord, under which Turkey would take back all asylum seekers arriving in Greece in what has become Europe's biggest migrant crisis since World War II, faces sharp criticism on several fronts.

"I am cautiously optimistic but frankly, more cautious than optimistic," said European Council President Donald Tusk -- who will chair the summit of 28 EU leaders on Thursday before they are joined by Turkish Prime Minister Ahmet Davutoglu on Friday.

"If we keep our cool we will achieve success," Tusk added after talks in Brussels with Ukrainian President Petro Poroshenko.
European Commission chief Jean-Claude Juncker meanwhile said on Twitter that he was "cautiously optimistic that we will conclude a binding agreement with Turkey" before he held eve-of-summit talks with Cyprus.

The summit comes as the UN said migrant arrivals in Greece alone had topped one million since January 2015, accounting for most of the 1.2 million people who sought asylum in the EU last year as they flee war in Syria, Iraq, Afghanistan and elsewhere.

Tusk stressed in his summit invitation letter that the parts of the migration deal that involve "re-energising" Turkey's long-stalled EU membership bid should also be "an opportunity to support the settlement talks in Cyprus. Only if this is possible, can we move forward here."

Cyprus has threatened to block the deal over long-standing disagreements with Ankara over the island, which has been divided since 1974 when Turkish troops invaded its north in response to an Athens-engineered coup attempt.

The only capitals that Tusk visited this week were Nicosia and Ankara, underscoring the seriousness of the threat.

Despite frantic efforts in Brussels to "rebalance" the deal in recent days, Tusk has freely admitted a major stumbling block remains the legality of the deal.

Under the key clause, in exchange for Turkey taking back all migrants arriving in Greece, the EU will resettle one Syrian for every Syrian readmitted on Turkish soil.

The UN and rights groups have warned that this amounts to an illegal mass expulsion of asylum seekers.

"An absolute priority is to ensure that our decisions respect both EU and international law," Tusk said in his letter.

A draft of the summit conclusions stresses that the one-for-one plan is a "temporary and extraordinary measure" and effectively capped at 72,000 migrants.

Proposals to give Turkey visa-free travel to the EU's Schengen passport-free zone by June have also worried several countries.

Critics have voiced unease over Turkey's arrests of journalists and academics, and an intensifying military campaign against Kurdish separatists.
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".

The draft conclusions say Turkey could get another 3.0 billion euros by 2018 in addition to an initial 3.0 billion agreed last November.

Merkel however told the Bundestag lower house of parliament on Wednesday that no shortcuts would be taken and that EU membership for Turkey was "not on the agenda now".

Juncker in a German newspaper interview said Turkey was "not ready" for membership and would not be for "another ten years".

Europe has been deluged by people fleeing war and misery since the start of last year, most hoping for new lives in wealthy Germany and Scandinavian countries.

The flow of people into northern Europe has been halted for now as eastern European countries have effectively shut down the Balkans route with coordinated border closures.

This, however, has caused a bottleneck of more than 40,000 migrants in Greece including thousands in dire conditions in makeshift camps on the border with Macedonia.