Calls for compassion for migrants suffering in North Africa and those attempting to reach Europe or die trying will be at the heart of Pope Francis's visit to Marseille this week.

The pontiff is making a two-day trip to France's second-largest city, a historic gateway for immigrants, where he is expected to insist on the causes of migration, from poverty to climate change, and urge greater tolerance.

He is also likely to address the horrors many migrants face in North Africa, from internment in brutal camps to being left by traffickers to die in the desert.

The visit comes as a surge in the number of migrant arrivals in Italy revives a bitter debate over how European countries manage asylum seekers.

"It represents a challenge that is not easy, as we also see from the news in recent days, but which must be faced together," Francis said after his Angelus prayer on Sunday in Rome.

"It is essential for the future of all, which will be prosperous only if it is built on fraternity, putting human dignity and real people, especially those most in need, in first place," he said.

The Marseille trip comes as Francis, 86, is in increasingly fragile health, saying on his return from Mongolia this month that papal voyages were not as easy as they used to be.

But he continues to travel widely, focusing on the smaller Catholic communities the Vatican calls the peripheries.

He is visiting Marseille first and foremost to take part in a meeting of Mediterranean-area Catholic bishops and young people.

"I will go to Marseille, but not to France," Francis said in August, despite the risk of offending French Catholics, in particular those on the conservative fringe, who think he goes too far with his messages of compassion for migrants.

The port city is a key destination for migrants from North Africa -- and is also home to some of the poorest neighbourhoods in Europe, many of which are plagued by drug trafficking.

"The problem that concerns me is the Mediterranean problem... The exploitation of migrants is criminal," Francis said in August.

More than 2,300 migrants have died so far this year attempting the Mediterranean crossing from North Africa, according to the UN.

Mediterranean 'cemetery'

The Marseille event, "Mediterranean Meetings", will look at themes including economic inequality, migration and climate change.

The pope is expected to speak to bishops active in North Africa in particular about the challenges there.

"The Mediterranean is a cemetery. But it's not the biggest: the biggest cemetery is in North Africa," Francis told reporters in August. "It's terrible. That's why I'm going to Marseille."

The pope will start at the Basilica of Notre-Dame de la Garde, a symbolic monument overlooking the city, for a prayer with the clergy on Friday afternoon.

That will be followed by a moment of meditation with representatives of other religions in front of a memorial dedicated to sailors and migrants lost at sea.

On Saturday morning, he will take part in the closing "Mediterranean Meetings" session at the Palais du Pharo overlooking the port, before leading a mass in the Velodrome stadium for about 57,000 participants.