Syrian government forces, backed by Russian air power, have launched an assault to capture the northwest, the last remaining territory held by rebels who are backed by Turkey.
With Russo-Turkish diplomacy in tatters, NATO-member Turkey has come closer than ever in the conflict to direct confrontation with Russia on the battlefield.
Opening the border could unleash a repeat of the migration crisis of 2015-2016, when more than a million people arrived by sea in Greece and crossed the Balkans on foot, until Turkey shut its frontier in a deal with the EU.
A million civilians have been displaced since December inside Syria near the Turkish border in desperate winter conditions, perhaps the worst humanitarian crisis of the nine-year war.
Turkey is already home to 3.7 million Syrian refugees and says it cannot take more.