Amnesty International accused Turkey on Friday of illegally forcing groups of Syrians to return to their conflict-torn country, saying the alleged expulsions showed the "fatal flaws" in a migrant deal agreed with the EU.
The claim by Amnesty comes just days before Turkey is due to start taking back migrants expelled from the EU under an accord reached last month.
The rights group said its research in the south of Turkey suggested the country was forcing around a hundred Syrians to return home on a daily basis.
Turkey -- which has taken in 2.7 million Syrian refugees since the conflict began in 2011 -- has always vehemently denied that any Syrian is forced to go home and insists its "open door" policy remains in place.
The government has yet to comment on the latest accusations from Amnesty.
"Turkish authorities have been rounding up and expelling groups of around 100 Syrian men, women and children to Syria on a near-daily basis since mid-January," Amnesty said.
Greece is due on Monday to start sending back to Turkey all migrants, including Syrians, who crossed the Aegean Sea illegally.
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EU leaders hope the deal will curb the influx of migrants that has plunged Europe into its biggest refugee crisis since the end of World War II.
But Amnesty said its revelations showed Turkey was not a "safe country" for refugees.
Turkish Prime Minister Ahmet Davutoglu on Thursday confirmed that under the scheme one Syrian refugee would be settled in Europe legally in return for every Syrian migrant sent back to Turkey from the Greek islands.
"In their desperation to seal their borders, EU leaders have wilfully ignored the simplest of facts: Turkey is not a safe country for Syrian refugees and is getting less safe by the day," said John Dalhuisen, Amnesty International's Director for Europe and Central Asia.
"The large-scale returns of Syrian refugees we have documented highlight the fatal flaws in the EU-Turkey deal," he added, lambasting the "EU's extended courting of Turkey".
He said it was "highly likely" that Turkey has returned several thousand refugees to Syria in the last seven to nine weeks, warning that those who are sent back under the EU-Turkey deal risk suffering the same fate.
An EU source told AFP in Athens on Thursday that 500 people were set to be sent back from Greece to Turkey on Monday, "barring a last-minute problem".