BRUSSELS – European Union’s (EU) leaders began a difficult summit on Thursday to push for a crucial agreement with Turkey to curb 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, had been hailed by German Chancellor Angela Merkel as the “first real chance” to end an influx of refugees and migrants. But the ambitious accord, under which Turkey would take back all asylum seekers arriving in Greece in what had become Europe’s biggest migrant crisis since World War II, faced sharp criticism on several fronts. “I am cautiously optimistic but, frankly, more cautious than optimistic,” said European Council President Donald Tusk who was to chair the summit of 28 EU leaders before they were joined by Turkey Prime Minister Ahmet Davutoglu on Friday. “If we keep cool, we will achieve success,” the EU president said after talks in Brussels with Ukrainian President Petro Poroshenko. Meanwhile, European Commission Chief Jean-Claude Juncker said on Twitter that he was cautiously optimistic that they would conclude a binding agreement with Turkey before he held talks on the eve of the summit with Cyprus. The summit came as the United Nations (UN) said that the 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 fled war in Syria, Iraq, Afghanistan and elsewhere. Tusk stressed in his summit invitation letter that the parts of the migration deal that involved “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,” he said. Cyprus has threatened to block the deal over long-standing dispute with Ankara. 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 had freely admitted that a major stumbling block remained regarding the legality of the deal. Under the key clause, in exchange for Turkey taking back all migrants arriving in Greece, the EU would resettle one Syrian for every Syrian readmitted on Turkish soil.