Pope Francis launched a stinging critique Tuesday of US President Donald Trump's deportations of undocumented migrants, describing it as 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.

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 defencelessness," he wrote.

Francis had warned as Trump returned to the White House last month that his pledge to carry out the largest deportation campaign in US history, by expelling millions of undocumented immigrants, would be a "calamity."

"This is not a minor issue — an authentic rule of law is verified precisely in the dignified treatment that all people deserve, especially the poorest and most marginalised," he wrote in his letter.

He added, "This does not impede the development of a policy that regulates orderly and legal migration. However, this development cannot come about through the privilege of some and the sacrifice of others. 

"What is built on the basis of force, and not on the truth about the equal dignity of every human being, begins badly and will end badly."

He welcomed the work done by Catholic clergy with refugees and migrants.

"I exhort all the faithful of the Catholic Church, and all men and women of good will, not to give in to narratives that discriminate against and cause unnecessary suffering to our migrant and refugee brothers and sisters," he wrote.