The Trump administration is intensifying measures to curb the flow of Central American asylum seekers crossing into the United States from Mexico, officials said on Monday, including sending more people back to Mexico to wait for their asylum claims to be heard by U.S. courts.

Department of Homeland Security Secretary Kirstjen Nielsen said the U.S. Customs and Border Protection agency will speed up the reassignment of 750 officers to parts of the border dealing with the largest numbers of immigrants, a shift the administration first announced last week.

U.S. President Donald Trump has threatened to close the border if Mexico does not stop a surge of people, often traveling as families from Honduras, El Salvador and Guatemala. Closing the border would potentially disrupt millions of legal border crossings and billions of dollars in trade.