The Trump administration on Friday will start returning some asylum-seeking migrants to Mexico to wait for their U.S. immigration court hearings, the first step in implementing a new border policy announced last month.

A U.S. official said under the program, starting at the San Diego border, selected migrants will be turned back after being processed by American immigration authorities and given a notice to appear in a U.S. immigration court in about a month, U.S. officials said.

A Trump administration official said the U.S. will determine how many migrants are sent back to Mexico each day, but didn’t indicate how many that would be.

Homeland Security Secretary Kirstjen Nielsen and Secretary of State Mike Pompeo announced the plan in response to the large number of migrants, mostly from Central America, crossing the border illegally or asking for asylum at ports of entry. Ms. Nielsen said in congressional testimony that change was needed because current policy allows asylum applicants to “disappear into the United States.”

In December, the Mexican government said it would allow Central American migrants back into the country while they wait for U.S. hearings. They will get humanitarian visas to stay and work. Officials there also said they have had multiple meetings with their American counterparts to discuss timing, technical details and logistics to ensure that asylum seekers have access to their lawyers.