U.S. officials at the southern border will begin sending some asylum applicants back to Mexico on Friday as the Trump administration implements new measures prohibiting migrants from waiting in the United States while their cases are processed.

The plan, announced by the Department of Homeland Security on Thursday night, follows high-level talks between the two governments late last year as U.S. border officials struggled to handle a surge of Central Americans fleeing violence and poverty. It will be introduced in California, at the San Ysidro port of entry south of San Diego, and eventually expanded throughout the nearly 2,000-mile border, a DHS official said earlier Thursday, speaking on the condition of anonymity because the plan had not yet been finalized.

According to a fact sheet distributed by DHS, “Certain aliens attempting to enter the U.S. illegally or without documentation, including those who claim asylum, will no longer be released into the country, where they often fail to file an asylum application or disappear before an immigration judge can determine the merits of any claim to prevent removal.”

In a statement, Homeland Security Secretary Kirstjen Nielsen called the new measure “unprecedented” and aimed at addressing “the ongoing humanitarian and security crisis at our Southern border.”