Pope Francis on Saturday arrived on the Greek island of Lesbos for a high-profile visit.
"This is a voyage marked by sadness, a sad voyage," the pope told reporters during the flight from Rome.
"We will witness the worst humanitarian disaster since the Second World War. We will see so many people who are suffering, who are fleeing and do not know where to go."
"And we are also going to a cemetery, the sea. So many people never arrived," the pope said.
Pope Francis took three families of Syrian refugees back to Rome after visiting the camp in Greece.
"The pope has desired to make a gesture of welcome regarding refugees, accompanying on his plane to Rome three families of refugees from Syria, 12 people in all, including six children," a statement issued by the Vatican said.
The island has also become the center focus for criticism of the EU's March deal ensuring refugees and migrants who travel to the Greek islands on boats operated by people smugglers are sent back to Turkey.
The EU-Turkey deal has resulted in new arrivals being stranded on Lesbos pending processing to determine which of them have a legitimate claim to refuge from conflicts like the war in Syria or from fear of persecution.
Accompanied by Orthodox Patriarch Bartholomew and Archbishop Ieronymos, the head of the Greek Church and Greek Prime Minister Alexis Tsipras, Pope Francis will spend five hours on Lesbos where they will visit the Moria processing centre.
The facility, currently housing around 3,000 people, has been described as a centre for "arbitrary detention" by rights groups.
Francis has framed his visit as an awareness-raising exercise and his spokesman insisted this week that its purpose was "strictly humanitarian and ecumenical, not political".
The pontiff, however, has demonstrated in the past that he is not one to mince his words and his spokesman reiterated "If he the pope has something to say he will say it."
'Open your doors'
Francis said earlier this week that the goal of his trip was "to show closeness and solidarity with the refugees as well as the citizens of Lesbos and to all the Greek people who have been so generous in their welcome".
The use of the term 'refugee' was not accidental. The former Jesuit priest -- and son of an Italian emigrant to Argentina -- has repeatedly said he does not accept the EU's distinction between those fleeing conflict and those fleeing poverty and starvation created by global economic inequalities.
And that line has been backed by Bartholomew, the Turkey-based leader of the world's Orthodox Christians, who said Europe as a whole must display the same generosity as the people of Lesbos.
"This offends God himself," he said. "The segregation of certain groups of people to the advantage of others does not reflect His desire."
During their visit to Moria, the religious leaders are due to have lunch with a small group of asylum-seekers followed by a larger meeting with around 250 refugees and brief discussions with Greek coastguards and local residents.
On a 2013 visit to Lampedusa, the Italian island which has witnessed several deadly sinkings of refugee boats off its shores, the pope made one of the defining speeches of his papacy, denouncing the "globalisation of indifference" which has allowed thousands to perish at sea.
Over one million people crossed clandestinely from Turkey to Greece in 2015 and some 150,000 have made the trip since the start of this year.