Pope Francis on Sunday urged the world to learn from history on the threat of nuclear war over Ukraine and choose the path of peace.

Looking back to the Second Vatican Council in the 1960s, "we cannot forget the danger of nuclear war that threatened the world at that time", the pope said during a canonisation mass in St Peter's Square.

"Why not learn from history? Even at that time there were conflicts and huge tensions, but the way of peace was chosen," the 85-year-old said.

US President Joe Biden said Thursday the world is facing nuclear "Armageddon", warning that Russian President Vladimir Putin may use his atomic arsenal as Russian troops struggle against a Ukrainian counteroffensive.

The Vatican said around 50,000 faithful attended the mass on Sunday celebrating the elevation to sainthood of religious figures Giovanni Battista Scalabrini and Artemide Zatti.

The pope used the opportunity to offer prayers for the victims of what he called a "mad act of violence" in Thailand.

A sacked police officer killed 36 people, 24 of them children, on Thursday in a gun and knife rampage at a nursery.

And in recalling the work of Scalabrini, a 19th-century Italian bishop who founded a community that serves migrants and refugees, Pope Francis returned to a recurring theme of his papacy.

"The exclusion of migrants is scandalous! Indeed, the exclusion of migrants is criminal, it makes them die in front of us," the pontiff said.

"And so today, we have the Mediterranean which is the largest cemetery in the world. The exclusion of migrants is disgusting, it is sinful, it is criminal."

He said that rather than opening the doors to migrants, "we exclude them, we send them away, to the concentration camps where they are exploited and sold as slaves".

Tens of thousands of migrants try to cross the Mediterranean each year in an attempt to reach Europe, but almost 25,000 have either drowned or gone missing since 2014, according to the UN's International Organisation for Migration.

Zatti was an Italian-born emigrant to Argentina who devoted his life to caring for the sick.