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.

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.

“We will not close those doors…. Why? Because the European Union should keep its promises.”He was referring to a 2016 deal with the EU to stop refugee flows in exchange for billions of euros in aid.

Erdogan’s comments were his first since 34 Turkish troops were killed on Thursday in northern Syria’s Idlib, where Moscow-backed Syrian regime forces are battling to retake the last rebel enclave.

There were skirmishes on the Turkish-Greek border at Pazarkule Saturday, as Greek police fired tear gas to push back thousands of migrants who hurled rocks at them, according to an AFP photographer at the scene.

“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.”

EU Commission chief Ursula von der Leyen expressed concern on the unimpeded flow of migrants from Turkey to the bloc’s external borders in Greece and Bulgaria.

“We averted more than 4,000 attempts of illegal entrance to our land borders,” government spokesman Stelios Petsas said after the meeting.

A Greek police source said migrants had started fires and opened holes in border fences. Police and soldiers patrolled the Evros River shores a common crossing point and issued loudspeaker warnings not to enter Greek territory.

The Greek coast guard said that from early Friday to early Saturday, 180 migrants reached the islands of Lesbos and Samos, crossing the eastern Aegean from the Turkish coast.