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.

The EU nations said in a statement that there are "serious deficiencies" to how Greece is protecting its border, which also is an external border of the whole Schengen zone of passport-free travel, and urged Athens to address them.

More than 1 million migrants have entered the EU since the beginning of 2015 and most have come through Turkey and into Greece as they make their way up to European nations like Germany and Sweden.

Several nations have imposed temporary border controls to deal with the crisis and many have criticized Athens.