Migrants behind the fence of a makeshift U.S. detention center in El Paso on Wednesday. (Sergio Flores for The Washington Post)

Immigration and Customs Enforcement is sharply curtailing the number of families it will hold at a detention center outside San Antonio, because the agency is unable to transport them there amid an influx of migrant families at the southern border, U.S. officials said Friday.

Beginning Monday, ICE will temporarily use part of the Karnes County Family Residential Center to detain about 700 adult women, with a smaller portion of the jail reserved for families. Earlier this month, the facility had 528 family members detained, but by Friday that had been reduced to 63 parents and children.

Officials expect the facility to return to exclusively detaining families after about three months.

“The current volume of family units crossing the Southwest border has overwhelmed ICE’s limited transportation resources to the point that ICE is currently only able to route a limited number of families apprehended at that border to the one other family residential center in Texas — the South Texas Family Residential Center in Dilley, Tex.,” ICE spokeswoman Danielle Bennett said in a statement.

The move comes as Homeland Security Secretary Kirstjen Nielsen and other officials are making urgent appeals to Congress to boost detention funding as a way to address the surge of migrants.

ICE said its adult detention space “is near capacity,” with 49,447 in custody as of last week. Nearly 75 percent of the 259,365 people booked into custody from October through March were apprehended at the border.

The shift reduces family detention space at one of ICE’s two largest family facilities and ensures that more migrants, after crossing, will be released directly into the country, where they will await an immigration court hearing — a process that can take months or years. Immigration and border officials already were releasing most families because of legal limits on how long they can detain children.

The head of Customs and Border Protection said this week that his agency is “reluctantly” releasing asylum-seeking migrants directly into the country — without ankle bracelet tracking devices or extensive vetting — because the U.S. border law enforcement apparatus is overwhelmed.

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Overall, more than 76,000 migrants were taken into custody — whether apprehended after crossing illegally or stopped at legal checkpoints — along the Mexico border in February, including a record number of families, many of them seeking asylum. This month, Nielsen said, CBP is on track to apprehend nearly 100,000 people.

In a letter to members of Congress on Thursday, Nielsen reiterated an “urgent” appeal for more money and authority to swiftly detain and deport Central Americans, including minors, to make room to process asylum cases. She warned that the Department of Homeland Security is “increasingly unable” to take “operational control of the southern border.”

“We are grappling with a humanitarian and security catastrophe that is worsening by the day, and the Department has run out of capacity, despite extraordinary ­intra-departmental and interagency efforts,” she wrote. “I am especially concerned about the level of families and unaccompanied children arriving at our borders and in federal custody.”

In addition to families, she said, hundreds of unaccompanied minors are spending too much time in crowded Border Patrol holding cells. Under federal law, children and teens traveling on their own are supposed to be swiftly transferred to Department of Health and Human Services shelters and then placed with a parent or guardian.

HHS is “taking steps to rapidly add thousands of shelter beds,” Nielsen said in the letter, first reported by NBC, but is “hitting peak capacity.”

HHS had about 12,000 minors in custody this week and was unable to immediately confirm Nielsen’s statement.

Families detained at the border end up in ICE custody, but space is limited to roughly 3,000 beds. Karnes is one of ICE’s largest facilities.

As of Friday, 63 family members were being held at Karnes, with 1,025 at the South Texas facility in Dilley, which is down about 400 people from earlier this month, and nine at a smaller facility in Berks County, Pa.

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Advocates for migrants have called for releasing them to await court hearings. ICE says it makes bond determinations “on a case-by-case basis.”

Border Patrol apprehensions remain well below their peak of 1.6 million in 2000, but the Trump administration says the nature of the flows has transformed from mostly adult Mexican men, who are easily deported, to asylum-seeking families and unaccompanied minors from Central America who have protections under federal law.

At a Michigan rally Thursday, Trump said the asylum system is swamped by claims from Central Americans who are aware that the government is unable to detain them, calling it a “big fat con job.” Trump has tried to get Congress to fund the construction of barriers along large stretches of the U.S.-Mexico border, and he has worked to narrow the ways foreigners can seek asylum in the United States. But many of the Trump administration’s efforts have been stymied by legal challenges.

Advocacy groups have argued that the Trump administration’s tough approach to the border violates federal asylum law. They say Central Americans are fleeing drought, hunger and violence at home and that the migrants should be treated as refugees.

And they have questioned Nielsen’s concern for child migrants, noting that she signed off on a policy last year that separated more than 2,700 children from their parents at the border, over the objections of experts who warned that it could inflict long-term emotional and physical harm.

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Nine months after Trump ended the policy, a federal judge is still trying to verify that all families have been reunited.

Rep. Bennie Thompson (D-Miss.), chairman of the House Homeland Security Committee, said in response to Nielsen’s letter that he would support funding to shelter migrants more humanely on the border, noting that some were being held this week under a highway overpass in El Paso. But he opposed her efforts to deport children and families more quickly.

“There is no legitimate reason for migrants to be held in horrid conditions or encamped under bridges,” he said in a statement.