The allegations come amid growing trepidation over a deal concluded in March between Turkey and the European Union that allows Greece to send refugees back to Turkey. European leaders, hoping to stop millions of migrants who are trying to reach their shores, have framed the agreement as an effort to dissuade refugees from making the perilous sea journey. But relief agencies and human rights groups have warned that several factors, including a continuing security crisis, make Turkey ill-equipped to carry out the deal or cope with large numbers of migrants.

There has also been consternation over the lengths European leaders have been willing to go to keep migrants away — including offering billions in aid to Turkey to conclude the agreement, while ignoring an increasingly harsh crackdown on President Recep Tayyip Erdogan’s opponents by the Turkish authorities.

“In their desperation to seal their borders, E.U. leaders have willfully ignored the simplest of facts: Turkey is not a safe country for Syrian refugees and is getting less safe by the day,” Amnesty’s director for Europe and Central Asia, John Dalhuisen, said in the report.

Also on Friday, the United Nations refugee agency warned that Greece’s overburdened asylum system lacked the capacity to process and register all the refugees seeking international protection, adding to concerns about the agreement. The agency called on the European Union and Turkey “to ensure all safeguards are in place before any returns begin.”

The allegations that Syrians were being forcibly returned highlighted the increasing hostility toward the refugees in the countries bordering Syria, particularly in Turkey and Jordan, which for years had been willing to absorb large numbers of refugees from Syria’s civil war, human rights groups said.