Turkish President Recep Tayyip Erdogan warned during a news conference on Tuesday in Istanbul that the Turkish parliament would block the landmark migrants deal with Europe if Ankara was not granted its key demand of visa-free travel. (Ozan Kose/AFP/Getty Images)

Growing tensions between Europe and Turkey over elements of a deal to end the refugee crisis are raising fears that the accord, signed by the two sides in March, may already be on the verge of collapse.

The latest sign of trouble came this week when Turkish President Recep Tayyip Erdogan warned European leaders that he would block the deal if the European Union refused to lift visa restrictions for Turks — one of the pact’s key provisions. He said that Turkey would not take any more steps to implement the agreement.

E.U. lawmakers have said Turkey must enact political reforms before they grant visa-free travel to Turkish citizens on the continent. Under the deal, Turkey also agreed to accept the return of asylum seekers whose applications are rejected in Greece and to crack down on the smuggling networks that facilitated much of the migration.

But even as Turkish and European leaders publicly spar over the details, critics have warned that the deal — hastily negotiated in the midst of the refugee crisis last year — has been flawed from the start. The agreement is based on the premise that Turkey, which hosts more than 2 million Syrian refugees, is safe for asylum seekers and that returning migrants to Turkish territory does not violate European or international law.

But some of the first deportations under the deal have already exposed abuses by Turkey’s government, and Turkish authorities have forcibly returned refugees fleeing war in Afghanistan and Syria, rights groups and E.U. lawmakers say. Last week, a Greek tribunal ruled that a Syrian national who had appealed his deportation from Europe could stay on the island of Lesbos. The court said there is no guarantee refugees will be provided full protection in Turkey.

“This decision goes to the heart of why the E.U.-Turkey deal was so deeply flawed to begin with,” Gauri van Gulik, deputy Europe director at Amnesty International, said in a statement.

Turkey signed the 1951 U.N. Refugee Convention — which forbids returning a person “in any manner whatsoever” to a place where his life or freedom would be threatened — but limited its commitments to refugees fleeing “events occurring in Europe.” Under the E.U. deal, however, refugees who have fled war in the Middle East and beyond should still be granted the right to apply for asylum in Turkey.

“Turkey is not safe for refugees,” van Gulik said. “Nobody else should be sent back under this deal.”

More than 1 million refugees and migrants reached European shores in 2015 in one of the largest mass migration movements since World War II. The conflicts raging in Syria, Iraq and Afghanistan have together displaced millions. And citizens of those three countries made up the bulk of asylum seekers in Europe last year, according to E.U. figures.

But as Germany and others opened their doors to refugees fleeing war, European leaders faced political backlash for what many in Europe saw as a growing and unmanageable crisis. With an average of 5,000 refugees and migrants arriving in Greece each day, according to the U.N. refugee agency, Europe scrambled to stem the flow.

Most of the refugees had crossed the sea from Turkey to Greece to get to Europe, and E.U. leaders needed to strike a deal with the Turkish government. European officials saw Turkey as the best hope for curbing migration, despite its increasingly repressive policies, experts said.

The E.U. offered more than $6 billion in funds to help Turkey, a member of the NATO military alliance, cope with its refugee population. And policymakers agreed that for every Syrian returned to Turkey under the E.U. deal, another Syrian refugee already residing in Turkey would be resettled to Europe.

But this was “not a deal that was made on the basis of mutual trust. It feels very political, and it was always going to be fragile,” said Elizabeth Collett, director of the Brussels-based Migration Policy Institute Europe.

“The people being returned [to Turkey] — they have no legal support, they’re in detention, they’re being kept in poor living conditions,” Collett said.

Of the roughly 8,500 refugees and migrants who have arrived in Greece since the deal went into effect March 20, fewer than 400 have been returned to Turkey, according to the Greek government.

“The management of the deal is inadequate . . . and the Greek government is reluctant to send anyone back who might have vulnerability,” Collett said. “The challenge now is predicting whether or not [the deal] will unravel.”

Collett’s concerns were echoed in a report released this month by a European parliamentary delegation that visited detention facilities in Turkey. The report, whose findings were based on a visit in early May, said that no one they interviewed was given the opportunity to ask for asylum, that access to a lawyer is “nearly impossible” and that the deportees were kept in “prison-style” detention.

“Deporting refugees to a place where they face such conditions is a disgrace,” said Cornelia Ernst, a European lawmaker from Germany, according to the report. “I cannot see how an agreement such as the E.U.-Turkey deal . . . can be legitimate or legal in any way.”

In Turkey, pro-government newspapers churn out anti-E.U. columns on a near-daily basis, calling on Erdogan to spurn a “hypocritical” Europe.

“The deal isn’t on hold,” a senior Turkish official said this week. He spoke on the condition of anonymity in accordance with government protocol. “Turkey maintains an open-door policy” toward refugees, he said.