Amnesty International has accused Turkey of illegally returning thousands of Syrians to their war-torn homeland in recent months.
The human rights group said Turkey has been expelling around 100 men, women, and children nearly daily since mid-January.
"EU leaders have willfully ignored the simplest of facts: Turkey is not a safe country for Syrian refugees," Amnesty's Europe and Central Asia Director John Dalhuisen said on April 1.
Turkey's Foreign Ministry denied Syrians were being sent back against their will, while a spokesman for the European Commission said it took the allegations seriously and would raise them with Ankara.
The news comes as Greece pressed ahead with plans to start deporting migrants and refugees back to Turkey next week.
Lawmakers in Athens on April 1 voted 169-107 to back draft legislation, fast-tracked through parliament, to allow the returns to start as soon as April 4.
The operation would see migrants and refugees who arrived on Greek islands after March 20 put on boats and sent back to Turkey.
The imminent deportations are backed by the European Union following its recent agreement with Turkey, and triggered more violence at detention camps in Greece.
Authorities on the Greek island of Chios said several hundred people pushed their way out of an overcrowded detention camp and staged a peaceful protest on April 1 in the island's main town, chanting "Freedom! Freedom!" and "Turkey No!"
More protests are planned on the island on April 2.
Greek officials did not respond to the criticism directly, but insisted the rights of detained asylum seekers were being protected.
"I assure you that we will strictly observe human rights procedures, not what people are inventing but what is required under the circumstances," Migration Affairs Minister Ioannis Mouzalas told parliament on April 1.
In Geneva, the United Nations refugee agency (UNHCR) urged Greece and Turkey to provide further safeguards for asylum seekers before the returns begin.
The UNHCR noted that conditions were worsening by the day for more than 4,000 people being held in detention on Greek islands.