The pope called for migrant detention centres in Libya to be cleared on Thursday, saying it was a "sin" to remain indifferent to the abuses suffered by refugees.

Pope Francis, who has made the plight of migrants a central theme during his pontificate, railed against indifference shown to those who face abuse in detention camps, spurring them to risk the dangerous voyage to Europe by sea.

He spoke before a group of migrants recently brought to Italy from a refugee camp on the Greek island of Lesbos.

Pope Francis (R) blesses a toddler during an audience with refugees who arrived in Italy from the Greek island of Lesbos
Pope Francis (R) blesses a toddler during an audience with refugees who arrived in Italy from the Greek island of Lesbos POOL / ETTORE FERRARI

"How can we fail to hear the desperate cry of so many brothers and sisters who prefer to face a stormy sea rather than die slowly in Libyan detention camps, places of torture and ignoble slavery?", the Catholic pontiff said, visibly moved.

"How can we remain indifferent to the abuses and violence of which they are innocent victims, leaving them at the mercy of unscrupulous traffickers?" he asked.

"Our ignorance is a sin."

Pope Francis unveiled a resin cross adorned with the life jacket of a migrant who died last year crossing the Mediterranean
Pope Francis unveiled a resin cross adorned with the life jacket of a migrant who died last year crossing the Mediterranean POOL / ETTORE FERRARI

Echoing calls made by the United Nations and humanitarian groups, the Argentine pope said the insalubrious and overcrowded Libyan camps -- where over 50 migrants were killed by an airstrike in July -- should be cleared and traffickers punished.

"Serious efforts must be made to empty the detention camps in Libya, evaluating and implementing all possible solutions," he said.

"We must denounce and prosecute traffickers who exploit and abuse migrants," Francis added, saying the migrant crisis "won't be solved by preventing them from landing."

Meeting the 33 migrants, the pope unveiled a resin cross adorned with a life jacket. The jacket had been worn by a migrant who died last year while crossing the Mediterranean, he said.

Francis travelled to Lesbos in April 2016 and upon his return, brought three Syrian Muslim families with him on his plane.

Lesbos is again this year the main port of entry to Europe for asylum seekers, after being overwhelmed by the arrival of about 100,000 migrants in 2015, according to the UN. Over 55,000 migrants have thus far arrived in Lesbos this year, the agency said.

On the same day as Francis' appeal, police on the Greek island of Samos fired tear gas against migrants protesting the grim conditions at their overcrowded camp, state agency ANA reported.

Libya