KASTANIES, Greece (Reuters) - Greek police fired teargas to push back hundreds of stone-throwing migrants trying to cross the border from Turkey on Saturday, as a crisis over Syria shifted onto the European Union’s doorstep.

Greece, which has tense relations with Turkey, accused Ankara of sending the migrants to the border post in an organized “onslaught” and said it would keep them out.

Turkey said on Thursday it would stop keeping hundreds of thousands of asylum seekers in its territory after an air strike on Idlib in neighboring Syria killed 33 Turkish soldiers.

Convoys of people appeared heading toward the land and sea borders of Greece, which was a gateway for hundreds of thousands of asylum seekers into Europe in 2015 and 2016.

“They (the migrants) didn’t come here on their own. They are being sent away and being used by (our) neighbor, Turkey,” Greek Public Order Minister Michalis Chrysohoidis told reporters near the northern Greek border town of Kastanies.

“Greece... faced an organized, mass and illegal attempt to violate its borders and it withstood this attempt,” government spokesman Stelios Petsas said, adding that by Saturday morning authorities had prevented more than 4,000 people from entering Greece.

The crisis is the first big policy test for Greek Prime Minister Kyriakos Mitsotakis, who was elected in July and promised a tougher stance on immigration.

Turkey hit back at the Greek accusations.

“Look who’s lecturing us on international law! They’re shamelessly throwing tear gas bombs on thousands of innocents piled at their gates,” Turkish Foreign Minister Mevlut Cavusoglu tweeted, along with pictures of hundreds of people sitting outside the Greek border post.

“We don’t have an obligation to stop people leaving our country but Greece has the duty to treat them as human beings!”

The European Union said it was supporting Greece - and its neighbor Bulgaria, which also shares a border with Turkey - in protecting the bloc’s borders, but also sought to placate Ankara.

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It expressed its condolences with Turkey over the deadly Idlib strike in a statement and said the bloc was ready to step up humanitarian support.

Nearly 1 million refugees and migrants crossed from Turkey to Greece’s islands in 2015, but that route all but closed after a EU-Turkey pact in March 2016. Under the deal, Ankara had agreed to help stem the flow of migrants crossing into Europe in return for billions of euros in EU aid.

“The EU is actively engaged to uphold the EU-Turkey (migration pact) and to support Greece and Bulgaria to protect the EU’s external borders,” European Council President Charles Michel, who chairs the 27 national leaders of the bloc, said after a call with Turkish President Tayyip Erdogan.

The head of the bloc’s executive European Commission, Ursula von der Leyen, said the EU was ready to involve its border agency Frontex to help control the land border.

‘TITANIC EFFORT’

Erdogan said on Saturday some 18,000 migrants had already crossed from Turkey into Europe. Speaking in Istanbul, he did not provide evidence for the number but said it would rise.

Bulgaria said it had seen no migrant inflows.

Earlier, a Reuters witness said there were about 500 people between the Greek and Turkish border posts, and beyond that on the Turkish side, hundreds more.

The International Organization for Migration put the number of people along the Greek-Turkish border at 13,000. As the sun set, it said, buses in Turkish cities were still being loaded with people bound for the border area.

Overnight, demonstrators hurled flaming pieces of wood at police in Kastanies, amateur footage filmed by a policeman on the scene, which was seen by Reuters, showed. Police fired teargas to keep people back.

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Kastanies is about 900 km (550 miles) north-east of Athens.

“We are making a titanic effort to keep our borders closed to such migration flows,” said border guard head Panagiotis Harelas, showing empty tear gas canisters that he said were hurled from the Turkish side. They had Turkish writing on them.

A Greek government official said that in the space of an hour, 20 hand-held teargas canisters were thrown from the Turkish side.

Greece has asked for an emergency meeting of EU foreign affairs ministers to discuss the migrant issue, a diplomatic source said.

‘TODAY IS GOOD’

Reuters video showed tens of people at a time climbing, and some throwing their bags over a three-meter (10-foot) fence covered with barbed wire along the Greek border in Turkey’s western Edirne province.

A group of Afghans with children waded across the fast-moving waters of the Evros river and took refuge in a chapel. They crossed into Greece on Friday morning.

“Today is good,” Shir Agha, 30, said in broken English.

Their shoes were caked in mud. Temperatures were close to freezing.

Greece had already said it would tighten border controls to prevent coronavirus reaching its Aegean islands, where thousands of migrants are living in poor conditions.

A Syrian man, who did not give his name, said they had been in the border area for two days.

“We have no food, look at these kids,” he said.