Turkish President Recep Tayyip Erdogan warned EU critics of his military offensive in Syria that he would "open the gates" to Europe for 3.6 million refugees in his country.
Mr Erdogan told activists from his ruling AK Party that the EU was wrong to calls the operation an invasion.
"Hey, EU, wake up," he said. "I say it again, if you try to frame our operation there as an invasion, our task is simple. We will open the gates and send 3.6 million migrants to you."
The offensive has been criticised by European states who do not accept Turkey's claims that it is acting in self-defence.
European officials have said any attempts by Turkey to resettle millions who have fled the conflict in Syria could fall foul of international laws against coercion.
"They are not honest, they just make up words," Mr Erdogan said. "We, however, take action and that is the difference between us."
The Turkish operation began on Wednesday evening, days after US troops were withdrawn from the north Syrian border regions.
The decision caused uproar in US Congress and among American allies as an abandonment of Syrian Kurds.
Ankara regards the Syrian Kurdish YPG militia as terrorists because of ties to the PKK group that has waged a decades-long insurgency in Turkey.
Kurdish representatives have condemned Washington's decision after years of fighting alongside American troops to defeat ISIS.
Federica Mogherini, the EU High Representative for Foreign Affairs, said the bloc urged Mr Erdogan to think again.
"The EU calls upon Turkey to cease the unilateral military action," Ms Mogherini said.
"Renewed armed hostilities in the north-east will further undermine the stability of the whole region, exacerbate civilian suffering and provoke further displacements."
French Foreign Minister Jean-Yves Le Drian said the offensive “must stop”, as the Europeans prepared for an emergency UN Security Council meeting on Thursday.
“It calls into question the security and humanitarian efforts of the coalition against [ISIS] and risks undermining Europeans’ security,” Mr Le Drian said.
Dominic Raab, the British Foreign Secretary, said he had "serious concerns" about Turkey's offensive.
"This risks destabilising the region, exacerbating humanitarian suffering and undermining the progress made against ISIS, which should be our collective focus," he said.
European officials say there has been an sharp rise in the number of arrivals, to more than 3,000 a month, from Turkey in recent months as tension with Mr Erdogan grew.
Under a 2016 agreement, Turkey committed to stop migrants reaching Greece. Greece was also allowed to send rejected asylum-seekers back to Turkey. In return, Ankara was promised $6.6 billion.
So far, half the funds have been given to Turkey.
