Homeland Security Secretary Kirstjen Nielsen has ordered Customs and Border Protection to deploy “emergency surge operations” to the southern border and dramatically expand a Trump administration program that requires migrants to wait in Mexico as they apply for asylum in the United States.

Nielsen on Monday ordered CBP to quickly reassign up to 750 blue-uniformed Office of Field Operations officers to assist Border Patrol agents with soaring numbers of Central American migrants crossing the U.S.-Mexico border illegally. She is considering an expansion of as many as 2,000 officers, a move that could slow pedestrian and motor vehicle traffic at legal ports of entry.

A Department of Homeland Security official said 545 officers had been reassigned as of Monday.

Nielsen also ordered CBP to immediately expand Migrant Protection Protocols, a new policy that returns asylum seekers to Mexico to await immigration court hearings in the United States. Three weeks ago, officials said they had sent 240 migrants back to Mexico since January. On Monday, Nielsen ordered them to return hundreds each day — a potentially labor-intensive effort because officials must interview migrants, provide them with an immigration court date and work with Mexico to return them across the border.

“The crisis at our border is worsening, and DHS will do everything in its power to end it,” Nielsen said in a statement. “We will immediately redeploy hundreds of CBP personnel to the border to respond to this emergency.”

The ramp-up comes as President Trump says he is considering closing the nearly 2,000-mile southern border, despite warnings that the closure would damage the economies on both sides while being unlikely to do much to affect illegal immigration.

CBP Commissioner Kevin McAleenan said last week that the immigration system had reached its “breaking point,” with thousands of families and unaccompanied minors surrendering at the border daily and packing holding cells, immigration jails and shelters. Most are funneled into an immigration court system already saddled with an 820,000-case backlog.

Officials expected more than 100,000 border apprehensions in March, an increase of more than 30 percent from the month before and the highest monthly total in a decade. Final numbers for March were unavailable Monday.

The Trump administration is urging Congress to increase its power to detain and deport Central American families and minors. Officials say those seeking asylum are virtually guaranteed release into the United States because of limited detention space and the massive immigration court backlog.

The Trump administration blames smugglers and weak immigration laws for the influx, while advocates for immigrants say Central Americans are fleeing hunger, poverty and violence in their native countries.

Meanwhile the number of families arriving continues to soar, and they are sometimes arriving in groups of 100 to 300 people, overwhelming Border Patrol. Last week, McAleenan said temporary Border Patrol stations were so crowded that CBP was “reluctantly” releasing migrant families directly into the United States instead of turning them over to Immigration and Customs Enforcement, which usually processes them first.

Trump has called on Mexico to halt the flows of Central Americans traveling through to the U.S. border. He also threatened to cut foreign aid to the migrants’ homelands in Guatemala, Honduras and El Salvador, for allegedly failing to stop them from leaving.

“Mexico must use its very strong immigration laws to stop the many thousands of people trying to get into the USA,” Trump tweeted Saturday. “Our detention areas are maxed out & we will take no more illegals. Next step is to close the Border! This will also help us with stopping the Drug flow from Mexico!”

Advocates and Democrats have said cutting aid probably would have the opposite effect, potentially spurring more mass migration as people flee difficult conditions.

Human Rights First, a migrant advocacy group, said returning migrants to Mexico “is doubling down on a failing immigration policy.”

“The move will endanger the lives of asylum seekers by returning even more of them to Mexico, despite the high risks of death and violence there,” said Eleanor Acer, senior director of refugee protection for Human Rights First. “Not only does this scheme violate U.S. law, but it gives a black eye to the United States’ proud legacy as a leader on the protection of refugees.” ‏

Nick Miroff contributed to this report.