With a go-ahead from the US Supreme Court, the Trump administration on Thursday began enforcing a radical new rule that would deny asylum to nearly all migrants arriving at the southern border — a move that spread despair among those fleeing poverty and violence in their homelands.

A spokeswoman for the Homeland Security agency that manages asylum cases said the policy will be retroactive to July 16, when the rule was announced.

The UN warned on Friday that a US Supreme Court ruling allowing Trump’s administration to implement significant asylum restrictions could have dire implications on people in urgent need of protection.

“We regret the impact that the implementation will have on asylum seekers,” UN refugee agency spokesman Andrej Mahecic told reporters in Geneva.

While the Supreme Court ruling is only temporary, Mahecic voiced concern over the impact implementing the Trump policy could have on people seeking protection while the case winds its way through the courts.

He stressed that “any person fleeing violence or persecution must be able to access full and effective asylum procedures and international protection.”

Asylum requests at the southern US border are often made by families and unaccompanied children saying they have fled endemic violence and poverty in their countries, and Mahecic insisted that many are “in urgent need of safe haven.”

The new policy would deny asylum to anyone at the US-Mexico border who passes through another country on the way to the US without first seeking asylum there. Late on Wednesday, the Supreme Court cleared the way for the administration to enforce it while legal challenges move forward.

Migrants who make their way to the US overland from places like Honduras, Guatemala and El Salvador would be largely ineligible for asylum, along with people from Africa, Asia and South America who try to get in by way of the US-Mexican border.

“Our Supreme Court is sentencing people to death. There are no safeguards, no institutions to stop this cruelty,” the immigration-assistance group Al Otro Lado said in a statement.

The Mexican government likewise called the court’s action “astonishing.” The effects of the new policy could fall heavily on Mexico, leaving the country with tens of thousands of poor and desperate migrants with no hope of getting into the U.S.

Acting U.S. Customs and Border Protection Commissioner Mark Morgan called the Supreme Court’s go-ahead a “big victory” in the Trump administration’s attempt to curb the flow of migrants.

In Tijuana, Dunea Romero, a 31-year-old Honduran woman, started to tear up at the thought of not being allowed to take refuge in the US. She said she packed a bag and fled her homeland with her two boys, ages 7 and 11, after hearing that her ex-husband, a powerful gang leader, was going to have her killed.

“I did this so I didn’t lose my life,” she said. “I didn’t want to leave my sons without a mother.”

A Nicaraguan who has been waiting in Ciudad Juarez to request asylum in the US could be among those who would be affected. He said he fled his home country after running afoul of the Nicaraguan government with many acts of civil disobedience.

He passed through Honduras on his way to the border, meaning he would have to return to that violence-ridden country under the new rule. He asked that his name not be used because a past news report quoting him led to retaliation against relatives in Nicaragua.

An estimated 45,000 migrants who have been turned back by the US government and forced to wait out their asylum requests on the Mexican side of the border under yet another new, more stringent Trump administration policy.

The new rule on asylum represents a shift in decades of US policy.

Agencies