Greece has blocked nearly 10,000 migrants trying to enter from the Turkey border over the past 24 hours, a Greek government source said Sunday. 

A massive influx of migrants swelled along the border over the weekend after Turkey's President Recep Tayyip Erdogan threatened to open its frontier to Europe. 

He is seeking to pressure EU governments over the Syrian conflict after an airstrike in Syria's Idlib province on Thursday killed dozens of Turkish soldiers. 

A Greek government source said Sunday its army and police had stopped thousands from entering its territory. 

"From 0600 (0400 GMT) Saturday morning to 0600 Sunday morning, 9,972 illegal entrances have been averted in the Evros area," the government source said, referring to the northeastern region along the Turkey border. 

Huge crowds tried to cross into Greece via the Kastanies Forest in the early hours of Sunday, the source said. 

Greek authorities said 73 migrants had been arrested, but added that they "weren't from Idlib, but from Afghanistan, Pakistan, Somalia". 

Some 13,000 migrants have gathered along the Turkish-Greek border, including families with young children, the International Organization for Migration said Saturday. 

Greek security forces are patrolling the Evros river shores -- a common crossing point -- and have issued loudspeaker warnings not to enter Greek territory.

On Saturday clashes erupted along the border, where Greek police fired tear gas at migrants who in turn lobbed rocks at officers. 

Erdogan threatened to open Turkey's gates for some of the 3.6 million refugees it is harbouring as a way to pressure EU countries over the conflict in Syria.

Turkey and Russia, who back opposing forces in the conflict, have held talks to defuse tensions after the air strike left 34 Turkish troops dead, sparking fears of a broader war and a new migration crisis for Europe. 

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.

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 Saturday.

EU border protection

The EU's border protection agency Frontex said Sunday it is on "high alert" on Europe's borders with Turkey as thousands of migrants seek to enter the bloc, adding it is deploying support to Greece.

The EU's commissioner for migration, Margaritis Schinas, tweeted separately that he has requested an extraordinary meeting of EU interior ministers to discuss the situation.

He added that there were "ongoing consultations" between European Commission chief Ursula von der Leyen, Greek Prime Minister Kyriakos Mitsotakis and EU Home Affairs Commissioner Ylva Johansson.

A spokeswoman for Frontex told AFP in a statement that "we... have raised the alert level for all borders with Turkey to high."

She added: "We have received a request from Greece for additional support. We  have already taken steps to redeploy to Greece technical equipment and additional officers."

Thousands of migrants are getting unimpeded passage across Turkey to the borders of European Union member countries Greece and Bulgaria, raising major concerns in the European Union.

Turkish President Recep Tayyip Erdogan said he "opened the gates" for them after dozens of his troops were killed in airstrikes in northern Syria by Damascus's Russian-backed regime. 

He is seen to be using the migration flow to try to pressure the EU and its NATO member states to lend him support in Turkey's military operation in Syria.

Frontex said it was monitoring closely the situation along the Greek and Bulgarian borders with Turkey and "we are looking into other ways of supporting EU countries bordering Turkey".

The statement added: "We are in close contact with Greek authorities regarding additional support we can provide in this rapidly evolving situation."

It said it was also monitoring the situation in Cyprus, an EU member state whose northern part is controlled by Turkey and only recognised by Ankara.

The agency noted that it was already staffing operations in Greece and Bulgaria. Its current biggest deployment is in the Greek islands, where it had 400 personnel, and a small contingent in the Evros region on the Greek-Turkey border. It currently has 60 officers in Bulgaria, it said.