OTTAWA -- Prime Minister Justin Trudeau added his voice Wednesday to the global chorus condemning the Trump administration's practice of separating migrant children from their parents at the border -- a practice the U.S. president abruptly reversed later in the day with the stroke of a pen.

Trudeau had been under pressure to condemn the so-called "zero-tolerance policy," under which asylum seekers who cross illegally into the U.S. are charged with federal crimes and separated from their children, who are detained in guarded, fenced enclosures.

He'd remained largely silent on the issue, saying only that he did not want to "play politics" on immigration policy. On Wednesday, however, his position shifted, just hours before Trump appeared to capitulate to political pressure by reversing course with an executive order.

"What the Americans are doing is unacceptable -- and it's not just me who has said it; all kinds of Americans, including Republicans, have said this is unacceptable," Trudeau told a news conference marking the end of the spring legislative sitting.

"We take actions based on facts, not on fears or worries.... It seems they want to change their approach. We hope they'll improve the system, but as far as we're concerned this situation cannot last."

Earlier in the day, Trudeau made a point of delivering a statement as he entered the weekly caucus meeting.

"What's going on in the United States is wrong," he said. "I can't imagine what the families who are living through this are enduring ... this is not the way we do things in Canada."

Trump's decision to reverse the practice marked a significant turnaround for a president who has made his hardline approach to immigration a cornerstone of his political identity, and who insisted that federal law gave his administration no choice but to separate families apprehended at the border.

Illegal migrants in the U.S. will continue to face the "zero-tolerance" policy, which includes detention and criminal prosecution, but the executive order will allow families to remain together while in custody, expedite their cases, and seek departmental help to house families.

In Canada, the Trudeau government was again under fire over the Safe Third Country agreement with the U.S., which requires would be asylum seekers to make their claims in the first "safe country" where they arrive.

That designation no longer applies to the U.S. in light of what's happening at the Mexico/U.S. border, the Canadian Council for Refugees argued Wednesday.

The government continues to monitor changes in U.S. immigration policy and the effects they will have on asylum seekers in the U.S., namely on their rights to due process in making asylum claims and appeal rights, said Immigration Minister Ahmed Hussen.

It's too early to say whether the recent American policy changes will infringe on those rights, he added.

"This is an evolving situation -- we have to, of course, take it into consideration and see how (the U.S. zero-tolerance policy) impacts the agreement," Hussen said.

"We can't make rash decisions."

But NDP immigration critic Jenny Kwan raised concern about the 2,300 children who have already been detained away from their parents by U.S. border security and how long it will take before they are reunited with family members now stuck within the U.S. criminal justice system.

"As long as Trump is in power, the U.S. will never be a safe country for asylum seekers," Kwan said.

The Canada Border Services Agency does detain some immigrants in Canada who are considered a flight risk or a danger to the public, and those whose identities cannot be confirmed. Canadian policy on child migrants allows detained parents to keep their children with them in holding centres if other alternatives cannot be found.

Concerns have been raised in the past over the number of migrant children detained every year by Canadian border officials. In 2016-17, 162 minors were detained or housed with their parents in holding centres, according to figures released by Public Safety Canada.

That number has been going down, Public Safety Minister Ralph Goodale said earlier this week. Last November, he issued a directive to the Canada Border Services Agency to keep children out of detention and keep families together "as much as humanly possible."

But several scientific studies on the effects of detention on child and adult asylum seekers have found any time in detention can be harmful.

A study by three medical researchers from McGill University submitted to the Commons Immigration committee in 2012 found short-term detention of adult asylum seekers leads to high levels of depression and post-traumatic stress disorder, while longer-term detention aggravates these symptoms.

The same study also found short term detention has a negative impact on children, with problems that include developmental delays, bedwetting, nightmares, separation anxiety, sleep disturbance, depression and suicidal behaviours.