WASHINGTON — The U.S. government said Tuesday it has expanded a program requiring asylum-seekers to wait in Mexico while their cases proceed through immigration court. They also reported that 240 migrants seeking asylum have been returned to Mexico under the program.

Homeland Security officials said the program is now at the Calexico port, about 120 miles east of the San Ysidro port near San Diego, where the program began in late January.

The Trump administration’s program marks a major shift in how the U.S. handles the cases of immigrants seeking asylum and fleeing persecution in their homeland. It is being implemented as border arrests soared in February to a 12-year-high and more than half of those stopped arrived as families, many of them asylum seekers who generally surrender instead of trying to elude capture.

Asylum-seeking families have typically been released from U.S. custody and allowed to settle with family or friends while their cases wind through immigration courts, which often takes years. Critics, including Trump, have said that amounts to “catch-and-release,” which administration officials want to limit with the new program. The program is also meant to deter those who make false claims; the number of asylum cases has skyrocketed and there is now a backlog of nearly 700,000 immigration court cases.

A lawsuit challenging the program was filed in California last month, though a federal judge hasn’t yet ruled on whether to halt the program while the legal case progresses.

According to memos obtained last week by The Associated Press, Mexican officials insist that no more than 20 asylum seekers are to be returned each day from San Diego to Tijuana, Mexico, on Monday through Saturday, underscoring the challenges the U.S. is facing in trying to quickly ramp up the border enforcement priority. U.S. officials must check if the asylum seeker has any felony convictions and notify Mexico at least 12 hours before they are returned.

Those who cross illegally must have come as single adults, though the administration is in talks with the Mexican government to include families.