Vatican City - Pope Francis launched a stinging critique Tuesday of US President Donald Trump, calling his campaign of mass deportations a "major crisis" that "damages the dignity of men and women."

In a letter to US bishops, he urged Catholics and others "not to give in to narratives that discriminate against and cause unnecessary suffering to our migrant and refugee brothers and sisters".

The Argentine pontiff, 88, has repeatedly defended the rights of migrants during his 10 years leading the Catholic Church, urging world leaders to be more welcoming to those fleeing poverty or violence.

"I have followed closely the major crisis that is taking place in the United States with the initiation of a program of mass deportations," he wrote Tuesday.

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He acknowledged "the right of a nation to defend itself and keep communities safe from those who have committed violent or serious crimes while in the country or prior to arrival."

But he wrote that "the act of deporting people who in many cases have left their own land for reasons of extreme poverty, insecurity, exploitation, persecution, or serious deterioration of the environment, damages the dignity of many men and women, and of entire families."

Deportation "places them in a state of particular vulnerability and defenselessness," Francis said, condemning "any measure that tacitly or explicitly identifies the illegal status of some migrants with criminality."

In a letter to Trump marking his return to the White House last month, the pope warned that the pledge to carry out the largest deportation campaign in US history would be a "calamity".