A Turkey-European deal to stop the flow of refugees into Europe has made seeking refuge in the continent "mission impossible" as vulnerable people stuck in camps in Greece suffer serious mental illnesses, according to a number of charities.

The International Rescue Committee (IRC), Norwegian Refugee Council (NRC) and Oxfam said on Thursday that the deal exposed refugees to risk and abuse, and accused Europe of setting a dangerous precedent.

"The EU-Turkey deal is playing roulette with the futures of some of the world's most vulnerable. It has become mission impossible for those who need it most to seek refuge in Europe," Panos Navrozidis, the IRC's country director in Greece, said.

In a separate statement on the same day, Save the Children and Doctors Without Borders, known by its French initials MSF, said refugees stuck in Greek camps, including children as young as nine, are cutting themselves, attempting suicide and using drugs to cope with the "endless misery".

The NGOs said anxiety, depression and aggression were on the rise in these camps, stressing that mental health was "rapidly deteriorating due to the conditions created as a result of this deal".

The EU-Turkey deal came into force on March 20, 2016, after more than a million refugees and refugees from Syria, Iraq, Afghanistan and beyond reached Europe in 2015, crossing over to Greek islands from Turkey.

Under the deal, anyone who crosses into Greece without documents can be deported to Turkey unless they qualify for asylum in Greece. But long asylum procedures and a huge backlog have stranded 14,000 asylum seekers on five Greek islands, double the capacity.

The European Union has hailed the deal as a success for stemming the tide of refugees to Europe through Greece.

An estimated 1.2 million people sought asylum in the EU in 2016, slightly fewer than in the previous year, the bloc's statistics office said on Thursday.