Turkey and Greece pledged Tuesday to work together to implement a "game-changing" proposal to ease Europe's migrant crisis, even as the United Nations and rights groups sounded grave concerns about the plan's legality.

European Union leaders, struggling to cope with the continent's worst migrant crisis since World War II, in principle backed a Turkish proposal to take back all illegal migrants landing on the overstretched Greek islands, at talks in Brussels on Monday.

Ankara also proposed an arrangement under which the EU would resettle one Syrian refugee from camps in Turkey in exchange for every Syrian that Turkey takes from Greece, in a bid to reduce the incentive for people to board boats for Europe.

EU officials have hailed the deal as a breakthrough, but the head of the UN refugee agency cast doubt on the legality of sending people back to Turkey, while Amnesty International said the plan "dealt a death blow to the right to seek asylum".

"As a first reaction 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," UNHCR chief Filippo Grandi told the European Parliament in Strasbourg.

He said the plan, which EU leaders hope to agree formally at a summit next week, did not offer sufficient guarantees under international law, adding that refugees should only be returned to a country if it could be proved that their asylum application would be properly processed.

Grandi called for refugees to be screened before being sent away from Greece "to identify highly at-risk categories that may not be appropriate for return".

Rights group Amnesty International said the proposal was full of "moral and legal flaws" and along with Human Rights Watch, challenged the idea that Turkey was a "safe country" to which migrants could return.

"The idea of bartering refugees for refugees is not only dangerously dehumanising, but also offers no sustainable long-term solution to the ongoing humanitarian crisis," Amnesty's Iverna McGowan said.

UN Secretary General Ban Ki-moon, speaking in Berlin alongside German Chancellor Angela Merkel -- a key player in the migrant drama -- said he was "concerned that many EU countries are adopting increasingly restrictive asylum policies".

European Commission chief Jean-Claude Juncker, who called the plan a "real game changer", insisted that it was "legally feasible".

Turkey is the main launching point for migrants making the dangerous crossing over the Aegean Sea to the Greek islands.

It already hosts 2.7 million refugees from the five-year civil war in neighbouring Syria, more than any other country.

After meeting his Greek counterpart Alexis Tsipras in the western city of Izimr on Wednesday, Turkish Prime Minister Ahmet Davutoglu said the plan would turn the Aegean from a "sea of despair in which people lose their lives" into a "sea of happiness and joy".

"In the decisions we have taken yesterday in the execution the cooperation of Turkey and Greece will be critical," Davutoglu said.

Tsipras said the situation in the Aegean Sea, where hundreds of migrants have drowned already this year making the perilous crossing in rickety boats, was a "shame and disgrace to our culture".

Greece and Turkey also renewed their commitment to a 2002 protocol on the readmission of migrants, which has rarely been activated before now, in the hope that its use could transform the refugee crisis.

For Turkey, perhaps the biggest gain in Brussels was the EU's agreement to bring forward to June visa-free travel to the EU's Schengen passport-free area for Turkey's 75 million people, provided that Ankara honours its promises.

Davutoglu further pushed for the opening of five more policy "chapters" in Turkey's long-drawn out EU accession process -- so far it has only completed one out of more than 30.

But securing a deal next week may still be difficult given the deep divisions that the migration crisis has sown in the bloc.

Hungary's hardline anti-migration Prime Minister Viktor Orban may veto the resettlement deal, while Cyprus said it remained opposed to accelerated EU accession talks for Turkey.

The Cyprus government -- not officially acknowledged by Ankara -- said Turkey must fulfil longstanding demands for official recognition and the opening of trade, ports and airports, before it would agree.

Deep rifts emerged in recent weeks over the main migrant route through the Western Balkans to wealthy Germany, after border restrictions by Austria triggered a domino effect of frontier closures that left tens of thousands of migrants stranded at the border between Greece and non-EU Macedonia.

In their closing statement, the EU leaders said that "irregular flows of migrants along the Western Balkans route have now come to an end".

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