[Refugees arrive by train at the transit center for refugees near northern Macedonian village of Tabanovce, before continuing their journey to Serbia, Friday, Feb. 12, 2016. In a dramatic response to Europe's gravest refugee crisis since World War II, NATO ordered three warships to sail immediately Thursday to the Aegean Sea to help end the deadly smuggling of asylum-seekers across the waters from Turkey to Greece. (AP Photo/Boris Grdanoski)]BRUSSELS (AP) — The European Union is acknowledging that the overall functioning of the Schengen area of borderless travel "is at serious risk" and says Greece must make further efforts to deal with the migrant crisis at its source.