Amnesty International says Turkey has rounded up scores of refugees and asylum seekers since September and transported them to detention centres where some have been mistreated or forcibly returned to Syria and Iraq.
In a report released yesterday, Amnesty said the mistreatment occurred "in parallel" with Turkish-EU migration talks and warned the EU that it risks being "complicit in serious human rights violations".
Turkey hosts the world's largest number of refugees, including 2.2 million Syrians.
This month, Turkey and the EU reached agreement aimed at curbing the flow of migrants into Europe. It includes an EU pledge to provide €3bn to help improve refugees' conditions in Turkey.
Turkey has denied Syrians were being forced back, and says all returnees are interviewed by UN staff. Amnesty has claimed refugees arriving in Turkey have been forced back to war zones and suggests EU money is being used for the operations.
According to the charity, the EU has allowed its funds to be used for equipment and infrastructure in Turkish facilities, from which refugees are being unlawfully pressured to return to countries like Syria and Iraq.
Refugees held in Turkey's Erzurum detention centre showed Amnesty staff the labels attached to beds and cupboards advertising the centre's funding under an EU pre-accession programme. EU officials in the Turkish capital Ankara confirmed to Amnesty that the six EU-funded open reception centres outlined in the draft action plan last month "will actually be detention centres".
Last month, the EU agreed to pay Turkey €3bn towards the cost of catering for Syrian refugees, with renewed momentum on talks for Turkey to join the EU. The two parties also agreed to fast-track visa liberalisation for Turkish citizens travelling to the EU in return for Ankara taking back deported migrants from EU member states.
As part of the EU-Turkey migration pact, Turkey also agreed to strengthen its western borders to stop the flow of migrants and refugees towards the EU.
But the Amnesty report has revealed that since September, the Turkish authorities have rounded up scores and "possibly hundreds" of migrants onto buses and transported them to isolated detention centres where they have been held.
Refugees told Amnesty they were detained without explanation or legal justification in a camp in Düziçi, in Osmaniye province, or in the Erzurum removal centre in the province for up to two months.
"When they put a chain over your hands and legs, you feel like a slave, like you are not a human being," said one 40-year-old Syrian man, who said he was held in the centre for a week.
The Turkish government has angrily rejected Amnesty's accusations, saying it "categorically denied" any Syrian refugee in Turkey had been forced to return to their conflict-torn home.