EU and Turkish leaders on Tuesday sought to turn into a lasting accord a plan on easing Europe’s migrant crisis hailed as a game-changer by Ankara and Brussels but swiftly criticised by the UN`s refugee chief.The day after agreeing the tentative plan following late night talks in Brussels, Turkish Prime Minister Ahmet Davutoglu was to discuss how it could work in a crucial meeting with his Greek counterpart Alexis Tsipras.The EU is wooing Turkey - used by over one million migrants in the last year as a springboard for reaching the bloc- as the key player in helping ease Europe`s worst migrant crisis since World War II.The talks in Brussels were expected to be tough but Davutoglu upped the ante by bringing additional Turkish demands to the table - such as accelerated visa liberalization - as well as offering more than EU diplomats expected.But after hours of talks, EU president Donald Tusk described the outcome as a "breakthrough" and said he would now work on the legal details to reach a final deal at a European summit in Brussels on March 17-18.A key pillar of the mooted deal was the unexpected offer by Ankara to take back every irregular migrant that crosses from Turkey to the islands of EU member Greece.In return, the EU would then resettle one Syrian living in Turkey on its territory for every Syrian migrant it takes back from Greece.The hope is that the plan will eliminate incentives for migrants to come to Greece by boat, but it ran into swift opposition from the head of the UN refugee agency Filippo Grandi."I`m deeply concerned about any arrangement that would involve the blanket return of anyone from one country to another without spelling out the refugee protection safeguards under international law," he told the European Parliament.Rights group Amnesty International said EU and Turkish leaders had "sunk to a new low" and ridiculed the notion that Turkey was a "safe country" to which migrants could return.Iverna McGowan, the head of Amnesty`s European Institutions Office, said "the idea of bartering refugees" was "dangerously dehumanising."- `Historic meeting` -Davutoglu was due to meet Tsipras later Tuesday in the Aegean port city of Izmir, with discussions expected to focus on Turkey taking back migrants who crossed from its shores to Greece."We will have a historic meeting where we will have a decision to update our readmission agreement so Turkey can take in all migrants who do not have a right to international protection," Tsipras said.Turkey and Greece signed a protocol over the readmission of migrants in 2002 but it has rarely been activated and its use could transform the refugee crisis.Tsipras said that Turkey had come to the table at the summit with "tempting proposals that surprised many people".