Calls for compassion for irregular 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 historical gateway for immigrants, where he is expected to insist on the causes of migration, from poverty to climate crisis, 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 irregular 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 the first place," the 86-year-old Pontiff said.

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 and asylum-seekers 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 a growing climate crisis.

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."

Around 5,000 police and other security forces will be mobilised for the trip, one welcomed by some residents -- though others have questioned whether Francis understands the challenges large numbers of migrants pose to the city.

"I don't entirely agree with the pope when he says 'we must welcome all migrants'," said Yvette Devallois, 69, who is active in her local parish.

"We welcome migrants, but still, we can't take in all the misery in the world."