Pope Francis and British Prime Minister Theresa May on Wednesday led global condemnation against the Trump administration's controversial policy of separating children from migrant parents at America's border with Mexico.
Pictures of dozens of children sleeping in cages and audio of children crying for their parents have emerged in recent days, triggering widespread criticism.
It comes after a 'zero-tolerance' crackdown on illegal immigration was brought in by United States Attorney General Jeff Sessions.
US immigration officials say 2,342 children were separated from 2,206 parents between May 5 and June 9.
Reacting to the Trump administration's 'zero tolerance' policy, Pope Francis said he supported US Catholic bishops who said it was 'immoral' and 'contrary to our Catholic values'.
"I am on the side of the bishops' conference," Francis said.
Prime Minister May said holding children 'in what appear to be cages [is] deeply disturbing' and 'wrong'.
Canadian Prime Minister Justin Trudeau said the US practice of separating migrant children from parents caught crossing the US-Mexico border illegally is 'wrong'.
"What's going on in the United States is wrong," he told reporters.
"I can't imagine what the families living through this are enduring. Obviously this is not the way we do things in Canada," Trudeau said.
Mexican Foreign Minister Luis Videgaray Caso said the practice was 'cruel and inhuman', and clearly violated human rights.
French far-right leader Marine Le Pen, who in the past has praised Trump's policy of curbing immigration, told French TV that she disagreed with splitting children from parents.
The Republican-controlled Congress is under pressure to change President Trump's policy.
Republicans are also scrambling to draft a new immigration bill to address the growing outcry.
President Trump told a group of House Republicans he would back any bill they passed, a White House spokesman said.
The president made clear that separating parents and children was 'certainly not an attractive thing and does look bad', Representative Tom Cole said.
However, Trump insists it is necessary to stop illegal border crossings.
On Tuesday, he said: "When you prosecute the parents for coming in illegally -- which should happen -- you have to take the children away."
Under the 'zero-tolerance' approach unveiled in May, all undocumented border crossers are criminally charged and jailed.
Migrant children cannot be held with their parents, and are kept in separate facilities maintained by the Department of Health and Human Services.
Under previous US administrations, undocumented immigrants caught crossing the border for the first time tended to be issued with court summonses.
But the Trump administration points out that most of those migrants never showed up for court.