It also complained that Turkey does not "systematically" share the legal status and whereabouts of readmitted Syrians, making monitoring all the more problematic.

The letter was citing figures from early November, noting that out of the 82 Syrians returned, it was only able to confirm 12 had been granted temporary status.

"Despite its best efforts, the UNHCR has not been able to contact the majority of the others," says the letter.

But Turkey's directorate general for migration management (DGMM) earlier this week told the European Commission that it had granted protection status to 131 out of 141 readmitted Syrians. The remaining 10 were sent back to Syria.

Although Turkey appears to be cooperating somewhat, administrative bottlenecks are making the UN's job difficult.

The UNHCR has to ask for permission to visit a refugee centre at least five days in advance, for instance.

After having made 16 requests to visit Duzici, it was granted access only 12 times. During those visits, it was only able to conduct individual interviews with around half of the Syrians.

Safeguards needed

Vincent Cochetel, who oversees the UNHCR's European operations, had already spoken out last September.

"We thought we had permission, we were not given access. For us that is an important aspect of the safeguards," he told this website at an event organised by European Policy Centre in Brussels.

When pressed over the blockade, EU migration commissioner Dimitris Avramopoulos at the time said Turkey was applying "adequate standards".

Turkey under the deal with the EU last March had "provided formal guarantees" that all the Syrian refugees readmitted to the country would be protected. Ankara had also introduced legislative amendments to its legal system.

"This appears to be sufficient protection equivalent to that foreseen by the Geneva Convention," the EU commission said on Wednesday.

Ankara's geographical restriction in the convention means only European nationals are granted full refugee status.

The EU says Turkey, under EU asylum laws, can be deemed a "safe third country" for non-Syrians and a "first country of asylum" for Syrians.

Around 838 people were returned to Turkey from Greece last year under the deal. Another 1,183 were sent back under a separate bilateral readmission agreement between the two countries.

No Syrian has been forced to return to Turkey under the EU deal, although one group of Syrians were allegedly tricked into being deported from Greece. The low number of returns has intensified pressure on Greek authorities to fast track applications and appeals.

Turkey hosts some 2.7 million Syrian refugees. Of those, around 10,000 have been given work permits since last September.