The European Union expects Greece and the European Border and Coast Guard Agency (Frontex) to conduct thorough internal investigations following reports that the EU border agency has been helping Greece push migrants back to open sea.

"We expect both Greek authorities and Frontex to investigate any such reports thoroughly and ensure full compliance with EU law," said Adalbert Jahnz, a spokesperson for the European Commission at a midday briefing.

"The commission is deeply concerned about the reports of pushbacks and other forms of noncompliance of EU law including safeguards of protection of fundamental rights and right to access asylum," Jahnz said.

He added that the commission is in contact with Frontex and Greek authorities, and Greece has started an investigation over the reports.

On Tuesday, Frontex also said that it had opened an internal inquiry over media reports that it was involved in illegal "pushbacks" of migrants.

"We are looking into the accusations leveled by several news organizations related to our activities at Greece's external borders," agency chief Fabrice Leggeri said in a statement.

But the Warsaw-based agency said that "so far, no documents or other materials have been found to substantiate any accusations of violations".

German weekly Der Spiegel on Friday reported that the EU border agency Frontex has been complicit in Greece's illegal practices against migrants in the Aegean Sea.

Earlier this month, nearly 30 nongovernmental organizations (NGOs) called on Greece to open an "urgent inquiry" into allegations that it was systematically pushing migrants back toward Turkey.

Numerous reports by journalists, researchers and witnesses have cataloged the way in which migrants and asylum-seekers have been forced back across the border to Turkey or left stranded in the Aegean Sea without aid by Greek coast guards, despite the conservative government's denials in Athens.

Last month, Oxfam and activist movement WeMove Europe filed a complaint with the European Commission, calling for an investigation into Greece's alleged violation of asylum-seekers' rights.

In June, the United Nations High Commissioner for Refugees (UNHCR), the International Organization for Migration (IOM) and the European Union jointly called for Greece to investigate the charges.

Der Spiegel published documents showing that Frontex units also observed and even backed these practices in recent months, in violation of international law.

Pushbacks are considered contrary to international refugee protection agreements. Under the principle of "non-refoulement," people should not be expelled or returned a country where their life and safety might be in danger due to their race, religion, nationality or being members of a social or political group.

The German weekly documented at least six incidents where Frontex units were involved in pushbacks near the islands of Lesbos and Samos between April 28 and Aug. 19.

While the border agency is required to rescue migrants, the Frontex vessels patrolling the area sped past the overcrowded, inflatable boats, creating dangerous waves to force them to return to Turkish shores.

A Frontex aircraft was also documented passing over migrants who were seeking help at sea but did not rescue them.

Der Spiegel's investigative reporting was also backed by Lighthouse Reports, Bellingcat, German public broadcaster ARD and Japanese broadcaster TV Asahi.

Turkey and Greece have been key transit points for migrants aiming to cross into Europe, fleeing war and persecution to start new lives.

Turkey, which hosts nearly 5 million migrants, has also accused Greece of large-scale pushbacks and summary deportations without access to asylum procedures, which is a violation of international law. It also accuses the EU of turning a blind eye to what it says is a blatant abuse of human rights.

The Turkish coast guard said it rescued over 300 migrants "pushed back by Greek elements to Turkish waters" last month alone. Citing what they say are credible reports, international rights groups have repeatedly called for an investigation.

Human Rights Watch (HRW) has accused Greece of summarily returning migrants across Turkish land and sea borders, citing interviews with asylum-seekers.

Other rights groups and refugee organizations, including the UNHCR, have repeatedly called on Greece to investigate what they say are credible reports and testimony of expulsions.

"The UNHCR is particularly concerned about the increasing reports, since March 2020, of alleged informal returns by sea of persons who, according to their own attestations or those of third persons, have disembarked on Greek shores and have thereafter been towed back to sea," the agency said in August.