Greek authorities say they have returned 13 Syrian refugees to Turkey under an agreement between that country and the European Union to stop the flow of migrants and refugees to Europe.

Police said the six men, three women and four children 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.

Under the EU-Turkey deal, those arriving clandestinely 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.