Turkey's President Recep Tayyip Erdogan on Saturday threatened to let thousands of refugees cross into Europe and warned that Damascus would "pay a price" after dozens of Turkish troops were killed inside Syria.
Around 13,000 migrants have gathered along the Turkish-Greek border, the International Organization for Migration (IOM) said late Saturday.
Earlier in the day, Greek police clashed with several thousand migrants, as they hurled rocks at security forces firing tear gas across the frontier.
Turkey and Russia meanwhile, who back opposing forces in the Syria conflict, held talks to defuse tensions after an air strike killed the Turkish troops, sparking fears of a broader war and a new migration crisis for Europe.
But Erdogan said he would let refugees travel to Europe from NATO-member Turkey.
"What did we do yesterday? We opened the doors," Erdogan said in Istanbul.
"We will not close those doors.... Why? Because the European Union (EU) should keep its promises."
"Look who's lecturing us on international law!" Turkish Foreign Minister Mevlut Cavusoglu tweeted. "They're shamelessly throwing tear gas bombs on thousands of innocents piled at their gates."
"I asked Mr Putin: 'What's your business there?'," Erdogan said. "If you establish a base, do so but get out of our way and leave us face to face with the regime."