Greek authorities said they have deported 13 Syrian refugees to Turkey under an agreement between the latter and the European Union to stop the flow of migrants and refugees to Europe.

The group comprises are six men, three women and four children, police reported. The immigrants were were flown Thursday from the island of Chios to Adana in Turkey on a flight chartered by Frontex, the European border patrol agency. It said those returned had not applied for asylum and returned voluntarily, AP reported.

Under the EU-Turkey deal, those arriving illegally on Greek islands from Turkey from March 20 face being returned to Turkey unless they successfully apply for asylum in Greece. The rate of returns has been slow after thousands applied for asylum. So far fewer than 500 people have been returned under the deal.

Problems in implementing the widely controversial EU-Turkey deportation deal have created a holdup of some 8,500 migrants on Greek islands, where brawls in overcrowded camps are common.The deal is the cornerstone of the EUs plan to curb the escalating aftermath of the crisis that has seen 1.25 million Syrian, Iraqi, Afghan and other migrants enter since 2015.