Greece should end its practice of pushing irregular migrants back from its borders toward Turkey and launch a probe into the allegations, Europe's top human rights watchdog has said.

In a letter to Greek ministers dated May 3 and published on Wednesday, the Council of Europe's Human Rights Commissioner said there had been "numerous credible allegations" since at least 2017 of asylum-seekers being illegally returned to Turkey or left adrift at sea but that Athens had simply dismissed them.

UNHCR, the United Nations' refugee agency, has also gathered information pointing to several dozen pushbacks since January 2020, said the letter from Commissioner Dunja Mijatovic.

"I urge you (Greece) to put an end to these practices and to ensure that independent and effective investigations are carried out into all allegations of pushbacks and of ill-treatment by members of security forces," Mijatovic said.

The Council of Europe is the guardian of the European Convention on Human Rights and the creator of the European Court of Human Rights.

In a response also published by Mijatovic's office, Greece said it had investigated the allegations and found them "largely unsubstantiated."

"The actions taken by the Greek authorities, at our sea borders, are being carried out in full compliance with the country's international obligations," Greek ministers were quoted as saying in their response to the council.

They said Greece had rescued thousands of people since the start of Europe's migrant crisis in 2015 and that officers had to do their job "against the backdrop of an unfavorable environment of intended misleading information emanating in most cases from the smugglers' networks."

EU member states were involved in pushing back some 40,000 migrants and refugees into Turkish waters, resulting in the deaths of over 2,000 individuals during the pandemic, according to an investigation by the Guardian last week.

The report noted that EU countries and their border agency Frontex systematically pushed back irregular migrants, including children, "using illegal tactics ranging from assault to battery during detention or transportation."

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

Pushbacks are considered contrary to international refugee protection agreements that say people shouldn’t be expelled or returned to a country where their life and safety might be in danger due to their race, religion, nationality or membership in a social or political group. Such actions prevent asylum-seekers from making claims for refugee status, and if practiced indiscriminately against a group of migrants it can constitute refoulement – a violation of EU human rights laws and the 1951 Geneva Convention.