The US is going to start forcing asylum-seekers who try to enter the US without papers to wait in Mexico for their asylum claims to be processed, instead of allowing them to enter the United States first.

A senior administration official described the policy as “one of the most significant border security developments in decades” on a call with reporters Thursday.

The policy change means that people who are trying to exercise their legal right to seek asylum will be barred from the US for months or even years while they wait for their asylum claim to come before a judge. It is the most sweeping development in Trump’s ongoing crackdown on people (largely from Central America, and disproportionately children and families) attempting to come to the US for asylum.

The new policy — which the Trump administration says was made unilaterally, without the involvement of the Mexican government — was announced by Homeland Security Secretary Kirstjen Nielsen and by the Mexican foreign ministry on Thursday. The countries are referring to it as “Migration Protection Protocols”; it is similar to a plan called “Remain in Mexico” that was being discussed between the administration and new Mexican President Andres Manuel Lopez Obrador before his inauguration.

Secretary Nielsen told Rep. Zoe Lofgren (D-CA) during a House Judiciary Committee hearing Thursday that the policy would not apply to children under 18 who come to the US without an adult. (The US’ treatment of non-Mexican unaccompanied children is governed by a specific law separate from the general asylum law.) However, a statement from the Mexican government on the policy said that it was cooperating with the US “for the benefit of migrants, in particular minors, whether accompanied or not.”

And on several occasions, Mexican authorities have physically blocked unaccompanied children from trying to seek asylum legally in the US at a port of entry. It is not clear whether the Trump administration will actively intervene with Mexico to make sure that unaccompanied minors are allowed to seek asylum in the US.

Once on US soil, people have a legal right to seek asylum. That can happen whether they enter at a port of entry (an official border crossing) in full accordance with US law, or enter between ports of entry and are apprehended by Border Patrol agents — committing the misdemeanor of illegal entry. The new policy would apply to both groups — those crossing illegally and legally.

The Trump administration has attempted to restrict access to asylum for people who enter the US between ports of entry, while telling people who arrive at ports of entry to wait. Now it will be able to turn both groups back entirely for the months or years it takes an asylum claim to come before a judge.

An administration official confirmed to Vox that asylum-seekers would have the opportunity to avoid being returned to Mexico if they demonstrated to a US official that they feared persecution in Mexico. It is not yet clear whether those claims would be held to the same standard that’s generally used to screen asylum seekers — a “credible fear” of persecution — or whether they will be held to a higher standard.

But the confirmation to Vox partly addresses the biggest concern raised by human-rights groups about the policy: that Mexico is not necessarily a safe place for asylum-seekers to be. Earlier this week, two teenagers from Honduras waiting to enter the United States and seek asylum were killed outside their shelter in Tijuana.

The policy relies on a provision in federal law that allows the US to return people who arrive in the US from “contiguous territories” back to those territories until their formal immigration-court proceedings have been completed.

In the past, Mexico hasn’t allowed the US to use this provision to return Central Americans and other non-Mexicans to Mexican soil. Now, they have relented. The deal will not apply to Mexicans seeking asylum in the US from Mexico, but will apply to all other groups.

Mexico will allow asylum-seekers to get “humanitarian visas, work authorization, and other protections,” according to Nielsen’s statement. It’s not yet clear whether they’ll be providing camps or other places for asylum-seekers to stay, or how they can guarantee the safety of asylum-seekers while in Mexico.

It’s not clear when exactly this policy will go into effect. The statement from Nielsen says that “Effective immediately, the United States will begin the process of invoking Section 235(b)(2)(C) of the Immigration and Nationality Act.” It is not yet clear what that process entails. A senior administration official told reporters Thursday that it would be rolled out in phases “in the coming days,” and that implementation would vary at different locations along the border.

Two front-line employees involved in apprehending and processing asylum seekers told Vox that they had not received any guidance about the new policy in advance of the public announcement.

The Mexican government is describing this as a “temporary, humanitarian measure,” according to the Associated Press. The US government, on the other hand, is describing it as the Trump administration’s solution to what it calls “catch and release” — the possibility that asylum seekers will disappear into the US rather than completing their asylum claims. Trump administration officials have described this as evidence of systemic fraud in the asylum system, despite suggestive evidence that many families are trying to complete their asylum cases and simply falling through the cracks.