Explainer: The new migration crisis at Europe's borders

Facing a potential wave of nearly a million people fleeing fighting in northern Syria, Turkey has thrown open its borders with Greece to thousands of refugees and other migrants trying to enter Europe, and has threatened to send "millions" more.

Greece responded by closing the land border, rushing in military and police reinforcements, and tried to stop migrant boats attempting the short but perilous crossing from the Turkish coast to its eastern islands.

Human rights organizations have called for urgent action to stop the situation from deteriorating further, warning of an unfolding humanitarian crisis.

Here is a look at the situation at the border, and at whether this the beginning of another migration crisis for Europe.

WHO ARE THE MIGRANTS GATHERED AT THE GREEK BORDER?

Turkey hosts 4 million refugees, some 3.6 million of them...