Migrants camp as they wait to cross the border at Turkey's Pazarkule with Greece's Kastanies, in Edirne. 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 organised "onslaught" and said it would keep them out.

This picture shows the Greek side of the Greece-Turkey border near Kastanies on Saturday. Sakis Mitrolidis/AFP

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

Convoys of people appeared heading towards 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) neighbour, Turkey," Greek Public Order Minister Michalis Chrysohoidis told reporters near the northern Greek border town of Kastanies.

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"Greece... faced an organised, 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.

A boy stands next to a tree as migrants in the background camp at Turkey's Pazarkule in Edirne. Reuters

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!"

Migrants carry wood to set up a fireplace at their camp near Turkey's Pazarkule border in Edirne. Umit Bektas/Reuters

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

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.

Greek soldiers guard at Kastanies border gate at the Greek-Turkish border on Saturday. Giannis Papanikos/AP

"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.

Reuters