Greece has blocked any new asylum applications for the next month after Turkey “opened the doors” for migrants to travel to the EU.
Greek officials earlier said they had stopped nearly 10,000 migrants crossing the land border with Turkey.
Prime Minister Kyriakos Mitsotakis said Greece had increased “the level of deterrence at our borders to the maximum.”
Turkey says it cannot deal with the amount of people fleeing Syria’s war.
Turkish President Recep Tayyip Erdogan said he was allowing migrants to try to get into neighbouring EU member states Greece and Bulgaria as of Friday.
His decision came after at least 33 Turkish soldiers were killed in air strikes in Idlib province in northern Syria this week.
Turkey is hosting 3.7 million Syrian refugees, as well as migrants from other countries such as Afghanistan – but had previously stopped them from leaving for Europe under an aid-linked deal with the EU.
But Mr Erdogan accused the EU of breaking promises made in 2016, when Ankara agreed to help shore up the EU’s south-western border.
The EU’s border protection agency Frontex said it was on “high alert” on Europe’s borders with Turkey.
Prime Minister Mitsotakis announced the suspension in asylum applications on Twitter on Sunday evening, and said Greece had invoked an emergency clause of an EU treaty “to ensure full European support”.
“The borders of Greece are the external borders of Europe. We will protect them,” he wrote, adding that he would be visiting the Evros land border with Turkey with European Council President Charles Michel on Tuesday.
“Once more, do not attempt to enter Greece illegally – you will be turned back,” he warned.