Asylum seekers say two men drowned after they were thrown into the sea by Greek authorities.
Greece's coastguard has been blamed for the drowning of at least two migrants amid further allegations of illegal pushbacks.
A Cameroonian asylum seeker has accused Greek authorities of throwing him into the sea last September off the coast of the island of Samos.
The man -- identified as Ibrahim -- said two other men were also cast overboard during a deportation operation to Turkey.
He told European media that Greek coastguards beat them "with their fists" before "throwing them overboard" without a lifeboat or lifejacket.
The bodies of the two other men -- from Cameroon and the Ivory Coast -- were later found by Turkish authorities and boats.
Ibrahim -- who has since filed an asylum application in Greece -- said he survived after managing to swim to the Turkish coast in the Aegean Sea.
He told reporters that he had boarded a dinghy alongside 35 others in Turkey, before arriving on the eastern Greek island.
There, several members of the group were subjected to violence by Greek coastguards and had their mobile phones and money confiscated, according to multiple sources.
The three men were later forced to board a boat before being thrown into the sea, Ibrahim said.
The International Organization for Migration’s issued a statement on Friday, saying it was “alarmed by mounting migrant deaths and continuous reports of pushbacks at the EU border between Greece and Turkey.”
NGOs have repeatedly accused Greece of ill-treatment in migrant camps, illegal actions at its border with Turkey, and denying migrants the right to asylum procedures.
Under European and international human rights law, people cannot be returned to a country where they would face torture, punishment or harm.
Athens has denied the allegations of illegal pushbacks and has labelled the claims "fake news" and "Turkish propaganda".
"In the absence of action by the Turkish authorities, the Greek Coast Guard continues to save the lives of thousands of men, women and children at sea every year," said Greek Migration and Asylum Minister Notis Mitarachi.
"Between 2015 and 2021, the Hellenic Coast Guard rescued more than 230,000 third-country nationals in danger at sea," he said in a statement.
"Greece protects the external borders of the European Union, in full compliance with international law and in full respect of the charter of fundamental rights".
Nineteen migrants were found frozen to death near the Greek border earlier this month, leading Ankara to blame Greece for the tragedy.
"Greece is not against legal immigration," said Notis Mitarachi. "We are against traffickers and all those who exploit human suffering, either for economic or political purposes."