Government officials in Greece have signaled that implementation of a migration agreement between the European Union and Turkey can only be implemented gradually, with key details still to be worked out on how migrants newly arriving from Turkey will be processed and returned.

Prime Minister Alexis Tsipras was meeting Saturday with ministers and senior officials involved in the migration crisis, hours after the deal was reached in Brussels to send back migrants arriving on the Greek islands to Turkey, starting Sunday.

Yiannis Balafas, the deputy interior minister, said swift screening procedures in the Greek islands would require additional staff promised by the European Union.