ATHENS (Reuters) - Stone-throwing migrants in a Greek detention camp clashed with police on Tuesday hours after two ferries shipped refugees back to Turkey under a disputed deal intended to stem the human influx into Europe.

A young man from Afghanistan sits on his bunk bed inside a tent at a camp for refugees and migrants in Schisto, near Athens, Greece, April 26, 2016. REUTERS/Alkis Konstantinidis

Plumes of smoke billowed from the Moria compound on Lesbos island that Pope Francis visited on April 16. Tensions simmering for days boiled over just after the Dutch and the Greek ministers responsible for migration toured the camp.

Garbage bins were set on fire, a police spokesman said, and migrants “were throwing stones and pieces of metal at police”. Earlier about 200 youths broke through a partition in the camp.

They were “reacting to their detention conditions and the returns to Turkey,” the spokesman said. Rights organizations have expressed misgivings about detention conditions on Moria, which holds about 3,000 people.

Just over 340 people have so far been returned to Turkey since April 4 under the accord agreed with the European Union in March after more than 1 million people fleeing war and poverty in the Middle East and beyond reached the continent last year.

On Tuesday, 13 people were deported from the island of Lesbos to the Turkish town of Dikili and five were ferried back from Chios to Cesme, police said. Most were Afghans and none had requested asylum in Greece, a government official said.

The United Nations High Commissioner for Refugees (UNHCR) and human rights groups have questioned whether the deal is legal or moral. They are also concerned about whether Turkey is a “safe” country for returnees. UNHCR does not currently have access to the Kirklareli camp returnees are sent to.

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The European Commission said on Tuesday it had been formally reassured by Turkey that it would grant access to asylum procedures to all asylum-seekers sent back from the bloc, a key outstanding element in the deal.

Turkey applies the Geneva Convention on refugees only to Europeans, offers limited protection to Syrians and no legal guarantees for other nationalities.

International law bans refoulement, or sending people back to a country where their lives or safety are at risk.

REQUESTS PILING UP

Under the deal, those arriving in Greece from Turkey after March 20 face being sent back if they do not apply for asylum in Greece or if their application is rejected.

In return, the EU will take in thousands of Syrian refugees directly from Turkey and reward Ankara with more money, early visa-free travel for its citizens and progress in negotiations to join the bloc.

Turkey Prime Minister Ahmet Davutoglu said on last week that Turkey would no longer need to honor the accord if the EU failed to ease visa requirements by June.

Brussels has said that Turkey fully meets only 19 out of 72 criteria for visa liberalization. On Tuesday, Turkey’s Deputy Prime Minister Mehmet Simsek said Ankara will meet criteria for visa liberalization by the beginning of May.

In Greece, the government has said authorities would start ruling on asylum applications in late April, but requests have been piling up and it has been criticized for being too slow to process them.

Giorgos Kyritsis, government spokesman for the migration crisis, said Athens was “not cutting corners (and) ... not delaying.” About 8,000 refugees and migrants are currently on Greek islands, having arrived after the deal was implemented.

So far under the deal, the International Organisation for Migration (IOM) has facilitated the resettlement of 350 Syrians from Turkey to European countries including in Austria, Denmark, and Germany, it said on Tuesday. It expects to resettle another 300 this week, mostly in France.