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."
He was referring to a 2016 deal with the EU to stop refugee flows in exchange for billions of euros in aid.
Turkey already hosts 3.6 million Syrian refugees.
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.
Turkey's Interior Minister, Suleyman Soylu, said nearly 50,000 migrants had left Turkey for Europe via the western province of Edirne, bordering Greece, in comments published in the official Anadolu news agency.
But the IOM said its staff had observed "at least 13,000 people gathered along the 212-kilometre (125-mile) long border".
"Thousands of migrants, including families with young children, are passing a cold night along the border between Turkey and Greece," it said.
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."
In 2015, Greece became the main EU entry point for one million migrants, most of them refugees fleeing the Syrian civil war.
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.
"Our top priority at this stage is to ensure that Greece and Bulgaria have our full support," she tweeted.
In Athens, Greek Prime Minister Kyriakos Mitsotakis held an emergency meeting Saturday to discuss the crisis.
"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.
One rubber dinghy arrived early Saturday in Lesbos carrying 27 African migrants, many of them women, who wept and prayed on their knees, said an AFP reporter.
The UN says nearly a million people -- half of them children -- have been displaced by the fighting in northwest Syria since December, forced to flee in the bitter cold.
Turkey said its forces had destroyed a "chemical warfare facility", just south of Aleppo.
Syria's state media denied the attack and the existence of such a facility.
Turkish drone strikes killed 26 Syrian soldiers on Saturday, an independent war monitor said.
The Britain-based Observatory for Human Rights said the strikes "targeted positions of the regime forces in the Idlib and Aleppo countryside".
The killing of Turkish troops by President Bashar al-Assad's forces -- backed by Russian air power -- has sent tensions between Ankara and Moscow soaring.
On Friday, Erdogan spoke by phone with his Russian counterpart, Vladimir Putin. Erdogan may travel next week to Moscow for talks, according to the Kremlin. But the Turkish leader remained critical on Saturday.
"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."
Erdogan spoke with French President Emmanuel Macron by telephone Saturday, the Turkish presidency said.
Seeking support from Europe after the Idlib casualties, Erdogan told Macron that Ankara wanted to see "clear and concrete support" from NATO "not only in words but in deeds".
He also warned that the humanitarian crisis would "deepen unless the regime's attacks are stopped in Syria".
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