The number of migrants who froze to death after being pushed back by Greece toward the Turkish border rose to 19 Thursday, as international organizations and Turkish senior officials slammed Greek authorities for the inhumane treatment of migrants.

On Wednesday, the Turkish interior minister had announced the death toll of migrants who were frozen to death in northwestern Turkey's border province Edirne after they were pushed back from neighboring Greece as at least 12.

In a new statement, the Edirne governorate said Thursday that Turkish authorities discovered more dead bodies after search and rescue activities in the Paşakoy village of Ipsala district, less than 10 kilometers (6.2 miles) from the Greek border.

Commenting on the incident, President Recep Tayyip Erdoğan slammed the European Union's border protection agency Frontex for cooperating with Greece and the bloc for turning a blind eye to the deaths of migrants. It is unacceptable to remain indifferent to such an incident, he said.

"Frontex is a useless EU organization that does nothing but provide assistance to Greece, which lets migrants die at sea, borders," Erdoğan told reporters in a news conference on Thursday before heading to Ukraine.

Erdoğan said he would bring up the issue of the alleged ill-treatment of migrants by Greece during every meeting he holds with world leaders.

"We will lead our struggle in front of the world," he said. "We will continue to be on the side of the oppressed. We consider this to be our humanitarian duty."

Migrants "pushed back" by Greek border forces and stripped of their clothes and shoes "froze to death," Interior Minister Süleyman Soylu had said on Twitter.

The European Union "is remediless, weak and void of humane feelings," said Soylu, adding that while Greek forces acted as a "thug" against people who had been made "victims," they were tolerant toward members of the Gülenist Terror Group (FETÖ), which was behind the 2016 defeated coup in Turkey.

Soylu shared several photos of the site where the irregular migrants were found, with the victims blurred.

The International Organization for Migration (IOM) said it was "horrified" by the reported deaths and that it would follow up the incident with the relevant authorities.

"Mounting reports of pushbacks against people on the move at some European borders and many parts of the world are extremely concerning and should be investigated and action taken," said Safa Msehli, a spokesperson for the IOM.

"We reiterate that such practices are prohibited under international law and should not happen under any circumstances," she said. "The obligation and primacy of saving lives and prioritizing the well-being and human rights of migrants are vital to the integrity of any border."

EU's Home Affairs Commissioner Ylva Johansson also expressed alarm at Turkey's claim.

"I just received the information and I must say I'm a bit shocked," she told Agence France-Presse (AFP) by telephone while attending a meeting of the 27-nation bloc's interior ministers in France.

"We have the Greek minister here, I will raise it with him and ask for clarification on this. This needs to be investigated of course."

Meanwhile, Greek Migration and Asylum Minister Notis Mitarachi issued a statement on Twitter regarding the incident, calling it a "tragedy."

"The deaths of 12 migrants on the Turkish border near Ipsala is a tragedy," he said.

Mitarachi, however, added: "These migrants never made it to the border. Any suggestion they did, or indeed were pushed back into Turkey is utter nonsense."

"Rather than pushing out unfounded claims, Turkey needs to live up to its obligations and work to prevent these dangerous journeys."

Turkey and Greece have been key transit points for migrants aiming to cross into Europe, fleeing war and persecution to start new lives. Turkey and many international human rights groups have accused Greece of large-scale pushbacks and summary deportations without migrants being given access to asylum procedures, which is a violation of international law. It also accuses the EU of turning a blind eye to this blatant abuse of human rights.

The uneasy relations between Ankara and Athens plunged into crisis in 2020 over conflicting claims on natural resources in the Eastern Mediterranean waters.

The conflict threatened to spill over into an all-out war when their naval warships collided in disputed circumstances near one of the contested energy fields. NATO then set up a telephone hotline aimed at addressing immediate security concerns.

But the two sides continued to accuse each other of trying to escalate tensions by staging air and sea exercises around a string of islands whose ownership has been under dispute for most of the past century.

These tensions have been compounded by Ankara's claims that Greece is violating international conventions by intercepting migrant boats in the Aegean and then sending them back to Turkey.

Turkey's coast guards say they have rescued more than 15,000 migrants pushed back by Greece last year. Rescuers say they have recovered the bodies of 15 migrants.

Athens denies violating international conventions and insists it is doing its duty to protect the EU's southeastern borders against illegal crossings. The EU has infuriated Turkey by largely supporting the Greek position. The EU's border and coast guard agency Frontex has also been involved in some Greek pushbacks of migrants.