Doctors Without Borders on Friday slammed the EU-Turkey deal to stem the influx of migrants to the bloc as a "historic abdication" of Europe's moral and legal responsibilities.

The medical charity, which goes by its French acronym MSF, voiced "profound concern" at the deal, under which Turkey has agreed to take back Syrian migrants landing on Greek islands in exchange for political incentives including billions of euros in aid and visa-free European travel for its citizens.

The agreement reached in March "effectively outsources caring for these people to Turkey," MSF chief Joanne Liu said in an open letter to EU member states and institutions.

"In an era of the greatest displacement of humanity in decades, this is a historic abdication of your moral and legal responsibilities," she said.

The Turkish agreement is the cornerstone of the EU's plan to curb a crisis that has seen 1.25 million Syrian, Iraqi, Afghan and other migrants enter since 2015.

But the deal was on Friday hanging by a thread after President Recep Tayyip Erdogan defiantly vowed Ankara would not adapt its counter-terror laws -- a key condition set by Brussels before granting Turks visa-free travel in the bloc.

The Turkish minister of European affairs, Volkan Bozkir, was set to meet Johannes Hahn, the commissioner for European enlargement negotiations, in Brussels on Friday morning.

Liu warned that the deal was "sending a troubling signal to the rest of the world: countries can buy their way out of providing asylum."

"If replicated by many nations, the concept of refugee will cease to exist," she cautioned, depicting a dystopian world in which "people will be trapped in warzones unable to flee for their lives, with no choice but to stay and die."

She also described Europe's accommodation of migrants and refugees stranded in Greece as "shameful".

"In camps on the Greek islands, there are virtually no safeguards in place," she said, pointing out that "women fear to go to the toilet once darkness falls... and men of all ages lose their dignity fighting over scraps of food."

Liu warned that the EU-Turkey deal risked putting aid agencies in the difficult position of deciding whether to "provide desperately needed aid in service of an anti-humanitarian policy that has the ultimate goal of border control."

"European countries, people are in need of your help and protection -- not just your money," she said, voicing outrage that Europe, the epicentre of World War II's massive displacement crisis, was "betraying the humanitarian principle of providing aid based on need alone."

"Is World War II so long ago that you no longer recall the basic human need to flee from violence and persecution when left with no other choice?" she asked, urging European leaders to "rise to the challenge" and help those in need.