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Last week, dozens of migrants who risked their lives crossing the Aegean Sea to reach Europe were returned from two Greek islands to Turkey — the first of thousands earmarked for return under a European Union deal with Turkey meant to stop the continent's massive refugee flow.

On Saturday, Pope Francis, Ecumenical Patriarch Bartholomew I and the head of the Church of Greece head to one of those islands to voice their solidarity with the refugees and migrants who have streamed into Europe fleeing war, poverty and persecution — a trip that could embarrass EU leaders already under fire from human rights groups over the deportations.

The Vatican said Thursday that Francis' five-hour visit to Lesbos was purely humanitarian and religious in nature, not political, and wasn't meant as a criticism of the deportation program. But spokesman the Rev. Federico Lombardi acknowledged that Francis has previously told Europe it had a "moral obligation" to welcome refugees, and that it was "evident" that the humanitarian crisis in Europe only exists because political solutions to regional conflicts haven't been found.

On Wednesday Francis said he and the Orthodox leaders intended "to express closeness and solidarity both to the refugees and to the Lesbos citizens and all the Greek people who are so generous in welcoming (refugees)."

The three religious leaders have been outspoken on the refugee issue, while the Church of Greece has mounted a massive aid effort for those flowing through Greece as well as caring for Greeks impoverished by their country's financial crisis. And while no official statements have directly linked their visit with the deportations, the timing is certainly significant.

The son of Italian immigrants to Argentina, Francis made his first papal visit outside Rome nearly two years ago to Lampedusa, a rocky Italian island near Africa and the main landing point for migrants smuggled across the Mediterranean from Libya or Tunisia. There, he denounced the "globalization of indifference" shown to migrants.

Bartholomew has been just as outspoken. In his Christmas message, he said the fact that children are forced to flee their homes in fear of their lives was "a disgrace for humankind."

For his part, Ieronymos on Tuesday blasted European countries' decision to build border fences to prevent refugees from entering and said Greece didn't have the capability to offer shelter to all those fleeing their homelands and shouldn't become "a warehouse of people."

The Lesbos visit, announced last week, was instigated by a suggestion from Bartholomew, the Istanbul-based "first among equals" in global Orthodoxy who has made forging closer ties with the Catholic Church a cornerstone of his tenure as patriarch and enjoys good relations with Francis.

The visit "will encourage and strengthen the thousands of suffering refugees, while inspiring the implementation of appropriate actions for the protection of the afflicted Christian communities as well as the proper response to the critical refugee situation," the patriarchate said.

Their presence on Lesbos, where more than half of all those heading to Europe first landed, will certainly be highly symbolic at a time of growing criticism of the March 18 EU-Turkey deal, which stipulates anyone arriving clandestinely on Greek islands on or after March 20 will be returned to Turkey unless they successfully apply for asylum in Greece.

For every Syrian sent back, the EU will take another Syrian directly from Turkey for resettlement in Europe. In return, Turkey was granted concessions including billions of euros to deal with the more than 2.7 million Syrian refugees living there, and a speeding up of its stalled accession talks with the EU.

Rights groups have blasted the deal as one trampling on the human rights of desperate people. And the Vatican has been just as clear in its criticism.

"It's not like Turkey is an example of liberty or democracy," Cardinal Antonio Maria Veglio, the Vatican official in charge of migration issues, told Vatican Radio, warning that those returned could be sent back to Syria or their families split up. "In addition, Turkey is getting something financial out of this accord. And it will do anything to get into the European Union."

The agreement, he said, treated refugees like merchandise.

"These poor migrants aren't things to be sent by mail, a kilo of merchandise: 'We'll take 80 kilos here and we'll send them there.' These are people!" Veglio said.

The religious leaders' first stop on Lesbos will be Moria, a refugee camp through which hundreds of thousands have passed on their way north. On March 20, the doors of the reception and registration center were locked and those living there prevented from leaving after it was turned into a detention center as part of the deal.

Francis, Bartholomew and Ieronymos will spend nearly an hour individually greeting about 250 refugees, and then have lunch inside a cargo container nearby with eight refugee representatives, the Vatican said. Short speeches and a joint declaration are planned.

They will then head to the island's capital and main port of Mytilene to meet residents, the small Catholic community, and lead prayers for the many refugees who drowned trying to get to Europe. Each of the leaders will toss a floral reef into the sea in memory of migrants who have died.

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Winfield reported from Vatican City.