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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. Officials also reported that 240 migrants seeking asylum have been returned to Mexico under the program.

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

The Trump administration’s program is 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 President Donald 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.

In response to Tuesday’s announcement, Mexico’s Foreign Relations and Interior departments said in a joint statement that they disagree with what they call a unilateral U.S. policy change. The statement said Mexico has worked with U.S. officials for “humanitarian reasons” and said a significant number of those returned have permission to stay in Mexico.

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 border enforcement. U.S. officials must check if asylum seekers have any felony convictions and notify Mexico at least 12 hours before they are returned.

The asylum seekers returned to Mexico must have come as single adults, though the administration is in talks with the Mexican government to include families.