Pope Francis on Sunday warned the Mediterranean had become the "biggest cemetery in Europe", as he remembered the migrants who died trying to reach the continent.

During his Angelus prayer, he noted a ceremony taking place in Sicily to mark the April 2015 tragedy when an estimated 800 migrants heading from Libya to Italy drowned when their boat capsized.

"This symbol of so many tragedies of the Mediterranean Sea will continue to challenge the conscience of all and encourage the growth of a more united humanity, which breaks down the wall of indifference," he said.

"Let us think about it: the Mediterranean has become Europe's largest cemetery."

Thousands of people hoping for a new life in Europe set off from north Africa each year, more often than not in overcrowded, leaky boats run by unscrupulous people smugglers.

More than a dozen boats arrived on Italy's tiny island of Lampedusa on Saturday, according to the Ansa news agency, with more than 1,200 migrants held in emergency facilities there.

But the central Mediterranean is one of the world's deadliest migration routes. It is patrolled by a handful of charity rescue ships, who complain that EU authorities are not doing enough to help their cause.

More than 500 people have died in crossings to Italy and Malta between January and mid-May this year, according to the UN's International Organization for Migration.