Leading US Catholic bishops have escalated their criticism of the Trump administration's immigration policies, calling new asylum-limiting rules "immoral" and rhetorically comparing the crackdown to abortion by saying it is a "a right to life" issue.

One bishop from the US-Mexico border region reportedly suggested "canonical penalties" - which could refer to withholding the sacrament of Communion - for Catholics involved in implementing the Trump policies.

The comments came as the US Conference of Catholic Bishops - the organising body of bishops - gathered for a biannual meeting in Fort Lauderdale. The topics of migration and asylum have long been a focus for the US church; more than 50 per cent of US Catholics under the age of 30 are Latinos.

The statements, including by the Conference's president, Cardinal Daniel DiNardo, archbishop of Galveston-Houston, came two days after Attorney General Jeff Sessions ruled that fear of domestic violence or gang violence aren't clear grounds for seeking asylum in the United States. Mr Sessions said asylum claims have expanded too broadly.

But the bishops said the ruling this week came on top of other Trump White House moves that they oppose. Those include ending a program that protected from deportation the "dreamers," young undocumented immigrants brought to the US as children, and reducing significantly the number of refugees allowed into the United States.

"At its core, asylum is an instrument to preserve the right to life. The Attorney General's recent decision elicits deep concern because it potentially strips asylum from many women who lack adequate protection. These vulnerable women will now face return to the extreme dangers of domestic violence in their home country. This decision negates decades of precedents that have provided protection to women fleeing domestic violence," Cardinal DiNardo said in a statement in his capacity as USCCB president.

The statement also condemned the "continued use of family separation at the US/Mexico border as an implementation of the Administration's zero tolerance policy. Our government has the discretion in our laws to ensure that young children are not separated from their parents and exposed to irreparable harm and trauma. Families are the foundational element of our society and they must be able to stay together."

The bishops' statement came a day after representatives of the biggest Protestant group in the United States - the Southern Baptist Convention - voted for a resolution that calls for protection of US borders along with a "pathway to legal status" and an emphasis on protecting family units.

According to the Religion News Service, Tucson Bishop Edward Weisenburger raised the possibility of implementing canonical penalties for Catholics "who are involved in this," referring to children being separated from their families at the border. Canonical penalties can range from denial of sacraments to excommunication, though Mr Weisenburger did not specify what he intended beyond referring to sanctions that already exist for "life issues," RNS reported.

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"Canonical penalties are there in place to heal," Mr Weisenburger said. "And therefore, for the salvation of these people's souls, maybe it's time for us to look at canonical penalties."

Efforts to reach Mr Weisenburger for details were not immediately successful late on Wednesday.

Some activists noted that it was rare for bishops to even talk about spiritual penalties in a political context, aside from warnings from some bishops to politicians who support abortion rights. John Gehring, a former USCCB staffer who is now a progressive faith advocate at Faith in Public Life, tweeted that "it's hard to overstate" the significance of Mr Weisenburger's remarks.

Cardinal Joseph Tobin of Newark, RNS reported, "proposed that a group of bishops be sent to the border to inspect the detention facilities where children are kept as a 'sign of our pastoral concern and protest against the hardening of the American heart.' Bishop Oscar Cantú of Las Cruces, New Mexico, suggested 'public gestures' such as prayer vigils in front of federal courthouses."

Cardinal Sean O'Malley, a confidant of Pope Francis who heads the Boston archdiocese, released a statement saying that while immigration policy is complex and must respect a variety of national and security needs, it is at its core "about people, young and old, alone or in families, often fearful and abandoned. Immigration policy is a moral question that cannot be separated from decisions of what it is right and wrong, of justice and injustice. It is about respecting and reverencing the dignity of the human person."

The new US immigration policies, Cardinal O'Malley said, "fail to communicate a willingness to address the reality of widespread human suffering, and in many cases imminent danger, with compassion and care."

"As a Catholic bishop, I support political and legal authority. I have always taught respect for the civil law and will continue to do so. But, I cannot be silent when our country's immigration policy destroys families, traumatises parents, and terrorises children. The harmful and unjust policy of separating children from their parents must be ended."

It wasn't clear what leverage the bishops might have with the Trump administration, whose faith advisory council is composed exclusively of evangelicals.

"When you have a social crisis of this magnitude, you have to look with two levels," said the Rev. Bryan Hehir, secretary for social services and health care for the Boston archdiocese. "You want to affect the policy, to make a counter-argument. Secondly I think from a historical perspective people will look back on this period of time and ask: 'Who said what?'"