WASHINGTON (AP) — Migrants heading to the southwest border to seek asylum in the United States will have to wait in Mexico until their claims are processed, under an agreement between the two countries announced on Thursday that will affect tens of thousands of people each month.

Only about 9 percent of people are actually granted asylum. The Trump administration says too many migrants make false claims.

"They will not be able to disappear into the United States," Nielsen said on Thursday in remarks before the House Judiciary Committee. "They will have to wait for approval. If they are granted asylum by a U.S. judge, they will be welcomed into America. If they are not, they will be removed to their home countries."

Discussions on the arrangement have been going on between the two countries for months, well before the new leadership took over in Mexico on Dec. 1. On Thursday, the Mexican foreign ministry said Mexico had agreed to it on a temporary basis for humanitarian reasons, and it would affect those "who entered that country or had been apprehended at border entry points, and who have been interviewed by that country's immigration authorities and who have received a court date to appear before an immigration judge."

American Civil Liberties Union attorney Lee Gelernt said the plan was illegal. "This plan cannot be done lawfully and will result in countless people in life-threatening situations."

The decision comes as the courts have blocked efforts to harden asylum rules.

More than 100,000 immigrants were caught crossing the U.S.-Mexico border in October and November. Nearly half of them were traveling in family groups that included children, according to statistics from U.S. Customs and Border Protection.

While the number of immigrants caught crossing the border illegally has fallen since the 1990s and early 2000s, U.S. authorities have been grappling in recent years with an increase in children traveling alone or with family.

It is not illegal to cross the border without a visa to apply for asylum. Immigrant advocates say violence in the Central American countries of El Salvador, Guatemala and Honduras is driving people north, and many are coming to seek asylum. Nearly 100,000 immigrants requested initial asylum screenings during the fiscal year ending in September, according to U.S. Citizenship and Immigration Services. Most of the cases were filed in Texas.

Trump administration officials say one of the major pull factors for migrants coming across the border is the idea that they can wait in the United States for months or even years as their asylum cases progress. They argue many disappear into the U.S. Forcing them to wait in Mexico will cut down on false asylum claims. The policy change applies only to migrants coming from countries other than Mexico, officials said.

Thousands of migrants have come up from Central America in recent weeks as part of caravans. President Donald Trump used his national security powers to put in place regulations that denied asylum to anyone caught crossing illegally, but a judge has halted that change as a lawsuit progresses.

Nielsen said in a statement the policy would be done legally.

"This will also allow us to focus more attention on those who are actually fleeing persecution," she said in a statement.

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Associated Press writer Mark Stevenson in Mexico City contributed to this report.