The Presidents of Turkey and Russia spoke over the phone on Friday, a day after Syrian Government airstrikes killed 33 Turkish troops, significantly ratcheting up tensions between Ankara and Moscow. It was the highest number of Turkish soldiers killed in a single day since Ankara first intervened in the Syrian conflict in 2016.

The development was the most serious escalation in the conflict between Turkish and Russia-backed Syrian forces and raised the prospect of all-out war with millions of Syrian civilians trapped in the middle.

NATO envoys held emergency talks at the request of Turkey, a NATO member, and scores of migrants began converging on Turkey’s border with Greece seeking entry into Europe after Turkey said it was “no longer able to hold refugees.” Turkish President Recep Tayyip Erdogan, whose country already hosts more than 3.5 million Syrian refugees, has long threatened to “open the gates” for millions of refugees eager to flee to Europe unless more international support was provided.

Refugees, meanwhile, headed to the land border with Greece, taking minibuses and taxis from Istanbul. Dozens waited at the Turkish side of the border gate at Pazarkule and dozens of others were in no-man’s land between the two countries.

Others headed to Turkey’s west coast to attempt to reach the Greek islands, a short distance away. Several rubber dinghy boats with groups of people clambering aboard were seen on Friday, heading for the island of Lesbos after apparently setting off from Ayvacik, northwest Turkey in broad daylight.