EL PASO, Texas (Reuters) - Huddled against a border fence on a bitterly cold morning in El Paso, Texas, a group of 60 Guatemalan migrants, around half toddlers and children, shouted for help: “We’re cold, we’re hungry, we need shelter.”

The group was trying to surrender to U.S. Border Patrol agents and claim asylum, but the agents were too busy herding other groups along the fence that stands about 100 yards (91 m) inside U.S. territory.

The 18-foot-high (5.5 meters) steel barrier is meant to deter illegal immigration. But its position inside the border has turned it into a destination for human smugglers trafficking large groups of asylum seekers fleeing poverty and violence.

The smugglers in recent weeks have shifted routes to El Paso from the remote Antelope Wells area of New Mexico, Border Patrol supervisory agent Joe Romero said.

Once undocumented migrants are on U.S. soil, the Border Patrol is obliged to arrest them for entering illegally. But migrants can claim fear of returning to their countries, allowing them to remain in the United States legally until an asylum hearing, which can take months or years.

The smugglers’ strategy exploits a weakness in the very border wall President Donald Trump has touted as a means to protect the United States from undocumented immigrants and illicit drugs.

The crowds in El Paso illustrate changing immigration patterns. As recently as 2015, the majority of undocumented border crossers were adult men from Mexico looking to disappear into the country and find work. Now the Border Patrol says about 85 percent of migrants arriving in the El Paso sector are Central American families and children seeking asylum.

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Gaspar Isom, 38, who was with his 16-year-old son Sebastian, said he chose El Paso for the relative safety of its sister Mexican border city, Ciudad Juarez.

“We were told other places were more dangerous to cross, they were controlled by the Zetas,” Isom said, referring to the Mexican cartel.

The pair were among close to 1,000 mostly Central American migrants who crossed into El Paso on Wednesday in the kind of surge the U.S. border has not seen in over a decade, Border Patrol data show.

El Paso is not alone in seeing an uptick. Over 268,000 undocumented migrants were arrested at the Southwest border from October through February, a near doubling over the same period a year earlier, to a 12-year high, according to government data released this week. Annual apprehensions remain well below the peak of 1.6 million in 2000.

Border Patrol officials say the El Paso fence, one of multiple sections of barrier built inside the border due to quirks of local topography, is successful in stopping migrants from scattering into El Paso.

But they acknowledge having a hard time keeping up with the numbers. El Paso sector Border Patrol stations reached capacity on Wednesday, and the group of 60 was finally picked up at 5 a.m. Thursday, after spending two nights sleeping by the fence, according to Dylan Corbett, who helps run a migrant shelter operated by El Paso’s Roman Catholic diocese.

Romero said the agency ran out of space to safely and securely transport migrants: “We have manpower shortages, our facilities are at capacity if not more.”