There are fears of a new surge of refugees arriving in Greece over the summer after it emerged the number of migrants being sent back to Turkey has fallen short of EU expectations.

Statistics released by Greek authorities show fewer than 400 have been sent back across the Aegean - because Athens deems the vast majority as being deserving of asylum.

Under a controversial deal between the European Union and Ankara in March, migrants not entitled to asylum are to be deported back to Turkey.

There are fears of a new surge of refugees arriving in Greece over the summer after it emerged the number of migrants being sent back to Turkey has fallen short of EU expectations

It went into effect on March 20 after the bloc was overwhelmed by the arrival of more than 1.25 million refugees from Syria, Iraq, Afghanistan and elsewhere since the start of 2015.

The Financial Times reports that some 8,500 people have arrived on Greek islands since the deal came into force, according to the country's migration co-ordination unit.

Athens officials have given the greenlight to just under a third of the 600 asylum applications from Syrians who have been assessed since the deal was brought into effect in March.

There are fears that the slow rate of returns could put the deal between the EU and Turkey at risk - sparking a fresh influx of migrants.

But the Financial Times quotes Maria Stavropoulou, the head of Greece's asylum service, as saying: 'We fully understand the [EU] concerns but if you look at it from the perspective of the rule of law, it is going exactly as it should.

Statistics released by Greek authorities show fewer than 400 have been sent back across the Aegean - because Athens deems the vast majority as being deserving of asylum

'We have many vulnerable people on the islands... a lot of very sick people. By law they are exempt from the return process.'

It comes as a UN human rights official said Greece must regulate and improve conditions for thousands of migrants - many of them children - detained in camps for weeks as they wait for asylum.

Francois Crepeau, the United Nations' special rapporteur on migrant human rights, said the 'perceived lack of information, plus overcrowding, creates confusion, frustration, episodes of violence, fear.'

In a written statement, Crepeau also urged Greece to 'renounce the idea' of detaining migrants for long periods except in cases of documented danger to others or flight risks.

There have already been flare-ups of violence in camps, mainly in the Moria facility on Lesbos island, where people increasingly desperate to be allowed to continue their journey to a better future in northern Europe

A UN human rights official has said Greece must regulate and improve conditions for thousands of migrants - many of them children - detained in camps for weeks as they wait for asylum

On Monday, a group of migrants set fire to garbage bins in front of the camp's asylum service. Police dispersed them with stun grenades, the state ANA agency said.

Last week, six migrants fed up with being stuck on the island of Chios tried to swim back to Turkey.

Lack of information is a major reason for the anxiety among the migrants, said Crepeau, adding that detaining children was flat-out unacceptable.

'Greece implements detention of children. Children should not be detained, period,' said the Canadian lawyer, who visited camps and detention centres in Athens, the islands of Lesbos and Samos, and Polykastro in northern Greece.

Crepeau's statement on Monday said the EU-Turkey deal was 'without mandatory value in international law' and of 'undetermined' legal basis.

There are over 54,000 people on Greek territory, including some 45,000 migrants who arrived before the EU-Turkey deal took effect and were stuck after other Balkan countries began closing their borders in mid-February.

Greece on Monday said it would begin preliminary registration of migrants who arrived prior to the EU-Turkey deal, and who wish to apply for asylum.

'The pre-registration exercise will take several weeks to conclude, but all those who arrived in Greece before 20 March, wishing to apply for international protection in Greece and are currently residing on the mainland will be able to pre-register,' the government said in a statement.