EL PASO, Texas (Reuters) - Hundreds of migrants are being held in a chain-link enclosure in El Paso, Texas, as the number of families crossing the U.S.-Mexico border in the city overwhelms U.S. Border Patrol facilities, the agency said on Thursday.

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The enclosure holds migrants crossing the border illegally in metropolitan El Paso as they wait to be processed at a nearby Border Patrol station, U.S. Border Patrol Agent Ramiro Cordero said by phone.

How long they remain in the enclosure, set up late last month below the city’s Paso del Norte International Bridge to Ciudad Juarez, Mexico, depends on how many migrants cross the border, he said.

“It could be a couple of hours, it could be more than that, it could be overnight, I can’t tell you, it’s just too many people for me to tell you an exact time or time frame,” Cordero said.

He said migrants are now crossing at an average of 570 people per day in the area, the highest rate in more than a decade, according to the Border Patrol.

Migrants at the enclosure are given thermal blankets and can get shelter, food, water and a medical evaluation, officials said. A Reuters photographer saw children sleeping outside in the enclosed area on Sunday night, when the low was around 47 degrees Fahrenheit (8 degrees Celsius).

“This is an inhumane and inexcusable way to treat people,” Taylor Levy, legal coordinator for El Paso migrant shelter Annunciation House, said by phone as she visited the enclosure on Thursday night.

She said migrants inside told her they had been there for between one and four days. She said she saw toddlers sleeping on the dirt and gravel beneath the bridge.

U.S. Customs and Border Protection Commissioner Kevin McAleenan told an El Paso news briefing on Wednesday the agency planned to set up temporary buildings to house migrant families, before a planned $192 million processing facility is built.

Those buildings have yet to be put up, according to Cordero.

More than 1,000 migrants were arrested in the El Paso sector on Monday, bringing the total number in CBP custody - including the enclosure - to almost 3,500 on Wednesday. The migrants are in facilities built for far fewer people and designed for single adults who once formed the bulk of arrests, McAleenan said.

Families and children now form the majority of apprehensions across the Southwest border, with a record 55,000 family units apprehended or encountered in March, McAleenan said.

El Paso migrant shelters are receiving around 700 people a day from immigration authorities, compared with a previous high of 2,000 a week in late 2018, said Dylan Corbett, director of El Paso’s Hope Border Institute which advocates for migrant rights.

“It’s not sustainable right now, that’s why everyone is really nervous, because this just can’t last,” Corbett said by phone.