Migrant children, many of whom have been separated from their parents under the "zero tolerance" policy by the Trump administration, are being housed in tents next to the Mexican border in Tornillo, Texas, U.S. June 18, 2018.

Canadian Prime Minister Justin Trudeau said on Wednesday the U.S. practice of separating immigrant children from their parents on its southern border was "unacceptable", even as he resisted pressure to suspend a bilateral agreement designating the United States a "safe" country for refugees.

Trudeau has been under increasing pressure to suspend the Safe Third Country Agreement, a 2004-era pact under which asylum seekers presenting at a land border crossing in either country are turned back and told to apply for refugee status in the first country they arrived in.

The agreement has been under fire for years in Canada from lawyers and advocates who argue the United States is not "safe" for refugees, and that its system is not analogous to Canada's.

"What is happening in the United States is unacceptable. I cannot imagine what these families are going through. Obviously this is not the way we do things in Canada," Trudeau told reporters at the parliament buildings on Wednesday.

The comments were by far the harshest by a member of the Canadian government, which is trying to prevent a trade war with the United States and is also struggling to control cross-border migration of its own.

More than 30,000 asylum seekers have illegally crossed the Canada-U.S. border since January 2017, many saying they were fleeing U.S. President Donald Trump's immigration policies.

Trudeau has come under increasing political pressure to condemn Washington for the practice of splitting up families and putting children in enclosed detention centers. As recently as Monday, he said he would not play politics with the issue.

This week, former federal Liberals as well as opposition politicians and rights groups called on the federal government to suspend the agreement.

"The United States is no longer 'safe' for asylum seekers... We should make crystal clear that we will not be complicit in his mistreatment of refugees," former diplomats Lloyd Axworthy and Allan Rock wrote in an op-ed in the Globe and Mail this week.

The Trump administration is facing intense criticism in the United States and abroad over its "zero tolerance" policy which can result in immigrant children being separated from their parents after they cross the U.S. border illegally.

Trump said on Wednesday signed an executive order that he said will keep families together but retain the "zero tolerance" policy.

A court challenge launched last year argues the Safe Third Country Agreement is discriminatory and violates Canada's Charter of Rights and Freedoms.

Recent developments, including child separations and U.S. Attorney-General Jeff Sessions' decision to disregard domestic violence as grounds for a refugee claim, only reinforce existing arguments against the agreement, said Janet Dench, head of the Canadian Council for Refugees, which is intervening in the case.