[An EU sign is seen as a migrant rests at the border crossing from Slovenia in Trnovec]Linking aid for countries in the Middle East and Africa to how they manage migration can create dangerous precedents, the top United Nations' refugee official warned the European Union on Monday. Under the most prominent of such collaboration, it promised Turkey an initial 3 billion euros in aid for Syrian refugees living there, accelerated EU accession talks and visa-free travel to Europe for its citizens. In exchange, Ankara has increasingly prevented people from leaving its shores for Greece, which was the main gateway to Europe last year but is now much less active.