Turkish President Recep Tayyip Erdogan warned EU nations on Tuesday that his country could send ISIS prisoners back to Europe, in response to sanctions over Cyprus.
Mr Erdogan also said before his visit to the US that Turkey would continue to repatriate foreign ISIS militants, even if their home nations declined to take them back.
His comments were in response to the EU's announcement on Monday of a system for imposing sanctions on Turkey for its unauthorised gas drilling in the Mediterranean off Cyprus.
"You should revise your stance towards Turkey, which holds so many ISIS members in prison and controls them in Syria," Mr Erdogan said.
On Monday, Ankara deported citizens of the US and Denmark who fought for ISIS, and made plans to send other foreign nationals to their home countries.
Turkish police accompanied one unidentified US citizen to the Kastanies border post on Monday and asked Greek police to let him enter. They refused.
"The Greek authorities did not allow the entry into our country of an American citizen of Arab descent, who had been arrested in Turkey and asked to enter Greece," police said.
"It is noted that from the audit conducted in databases of Greek and co-operating countries, nothing came to the detriment of the individual, who initially stated that he did not wish to enter Greece and then returned to Turkey, always accompanied by the Turkish policemen.
"This afternoon, however, the American of Arab descent went alone this time to the same frontier station and asked to enter Greece."
Last week Interior Minister Suleyman Soylu said Turkey was not a "hotel" for ISIS fighters.
Mr Soylu criticised western nations for their reluctance to take back citizens who had joined the extremist militant group as it sought to establish a "caliphate" in Iraq and Syria.
He said last week that about 1,200 foreign IS fighters were in Turkish prisons and 287 members, including women and children, were recaptured during Turkey's offensive in Syria.
Several European countries, including Britain, have stripped ISIS fighters of their nationalities to prevent their return.
Mr Erdogan slammed the EU's decision on Cyprus gas, saying Turkey was acting in line with its rights based on international law.
"Hey, EU, know this: Turkey is not one of those countries you have come to know until now. We are a country that sits at the negotiating table with you," Mr Erdogan said.
"These negotiations may suddenly end."
The EU relies on Ankara, which hosts more than 3.5 million refugees, to stop the arrival of migrants into Europe after a 2016 agreement to seal off the Aegean sea route.
Mr Erdogan has repeatedly warned that Turkey will allow refugees to travel to Europe unless it receives aid from other countries.
"You may take this lightly, but these doors will open and these Daesh members will be sent to you," he said on Tuesday. "Do not try to threaten Turkey over developments in Cyprus."
The island of Cyprus was divided in 1974 after a Turkish invasion sparked by a brief Greek-inspired coup.
Since then, several peace efforts have failed and the discovery of offshore resources has complicated the negotiations.
EU ties with Nato ally Turkey have meanwhile worsened after years of stalemate on Ankara's bid to join the bloc.
With a worsening record on human rights after a failed coup in 2016, many EU states say Turkey does not meet democratic criteria to join.
The decision to impose economic sanctions on Ankara follows a separate move last month to stop arms sales to Turkey over its offensive against the Kurdish YPG militia in north-east Syria.
Turkey's western allies have said the offensive could hinder the fight against ISIS, but Ankara rejected the claims.
