OTTAWA -- Prime Minister Justin Trudeau says he will not "play politics" over immigration policies when it comes to the controversial U.S. practice of charging and separating illegal migrants from their children when they cross the border into the United States.

The U.S. government is under fire for its "zero tolerance" policy -- including from the human rights chief of the United Nations Human Rights Council, which condemned the practice Monday as "unconscionable." Amnesty International also weighed in, describing children being torn from the arms of their parents and placed into "cages" as "nothing short of torture."

But Trudeau would not offer an opinion on the controversial policy, saying his role as prime minister is to stand up for Canadian values but also to maintain a constructive relationship with the U.S.

"What we will not do is play politics with this," the prime minister said. "We understand how important it is to be firm and unequivocal as we protect and support human rights around the world. And we will continue to do that both by example and by engagement with the world."

U.S. Attorney General Jeff Sessions announced the so-called zero tolerance policy in April as a response to a spike in illegal border crossings by asylum seekers in the United States.

As part of the policy, all irregular or undocumented migrants to the U.S. are being referred for federal prosecution and their children are taken and placed in holding facilities. Nearly 2,000 children were removed from their parents in April and May.

In one of several tweets published Monday, U.S. President Donald Trump defended the practice, saying children are being used by criminals to illegally enter America.

NDP immigration critic Jenny Kwan urged Trudeau to respond by suspending Canada's Safe Third Country agreement with the U.S.

"If there was ever a time for Canada to step up, the moment is now," Kwan said.

"It's clear to me the United States is no longer a safe country for asylum seekers. It is time for Canada to step up and suspend the Safe Third Country agreement and to rally the international community to action and to deal with Trump."

Canada's immigration laws require ongoing monitoring of the domestic asylum system in the U.S., and that assessment is currently being done to determine if the U.S. policy change will affect its designation by Canada and the UN as a safe country, said Immigration Minister Ahmed Hussen.

But before any changes to the Safe Third Country agreement could be made, Canada must first determine what effects on migrants will be, he added.

"As some of these changes are taking effect, we will see the impact that they have on due process, on appeal rights, on the ability for asylum seekers to actually make a claim and see whether the United States continues to meet its international (obligations)."

Public Safety Minister Ralph Goodale said children of immigrants and refugees are detained in Canada only as a last resort and new measures will soon be rolled out to offer alternatives to detaining migrants.

Concerns have been raised in the past over the number of migrant children detained every year by Canadian border officials. Last year, 162 minors were detained or housed with their parents in Canadian immigration holding centres. That number has been going down, said Goodale, who added that 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."

The Canada Border Services Agency holds people who are considered a flight risk or a danger to the public, and those whose identities cannot be confirmed. While alternatives are always considered first, some parents prefer their children to be housed with them.

"Obviously anyone looking at the human images (from the U.S.) would be very, very concerned," Goodale said.

"Children are very precious creatures and we all, I'm sure, need to have their safety, their security, their well-being first and foremost in our minds and that is what lies at the very basis of Canadian policy."

Josh Paterson, executive director of the British Columbia Civil Liberties Association, said while any detention of child migrants should only be used as a last resort, Canada's policies on how it deals with children of detailed asylum seekers are the opposite of the current U.S. practice.

"Canada's policy aims at precisely preventing what's happening in the United States, which is the separation of children from their families," Paterson said.

"(Canada) places family unity and the well-being of children first as core tenets of the program... it is better. It's in a completely different league."