Refugees in Turkey headed towards European frontiers on Friday after an official declared that borders had been thrown open in response to the escalating war in Syria, a day after 33 Turkish soldiers were killed by Russian-backed Syrian government troops.

European officials rushed to respond to a direct threat to reverse an agreement with Turkey that halted the migration crisis of 2015-2016, when more than a million people arrived by sea in Greece and crossed the Balkans on foot.

Moscow and Ankara traded blame over the strike in northwest Syria, the deadliest attack suffered by the Turkish army in nearly 30 years. Turkish financial markets plunged over the prospect of the country being plunged far more deeply into a new escalation of the nine-year-old war across the border in Syria.

“We have decided, effectively immediately, not to stop Syrian refugees from reaching Europe by land or sea,” a senior Turkish official told Reuters on condition of anonymity.