U.S. authorities have begun using aircraft to move migrants to less-crowded areas for processing, while asylum-seeking families were being bused north to Colorado to alleviate the strain on overwhelmed shelters along the border in Texas and New Mexico.

Several dozen migrants arrived by bus early Monday in Denver as part of the effort to help crowded shelters in El Paso and neighboring Las Cruces, New Mexico, where one facility reported running low on food.

Non-government organizations in Las Cruces were growing weary and overwhelmed, said Claudia Tristán, spokeswoman for Democratic New Mexico Gov. Michelle Lujan Grisham, whose office is aiding the transfers.

"They're experiencing a bit of burnout, particularly with the volunteers that have been dealing with this," Tristán said.

Unauthorized border crossings have surged since the start of this year, with El Paso being one of the busiest regions. In April alone, the Border Patrol apprehended nearly 99,000 people for crossing illegally, with more than two-thirds being unaccompanied children and adults traveling with children.

About 4,500 migrants passed through Las Cruces in the past month, making it likely that bus trips will continue to Denver, where migrants can use the robust bus and airline services to reach relatives and sponsors throughout the U.S. while they await immigration proceedings.

Meanwhile, Customs and Border Protection is planning to build a second tent facility at its temporary processing site in Donna, Texas. The first tent opened last week and already is about 100 people over its 500-person capacity, said Carmen Qualia, executive officer of operations for CBP in the Rio Grande Valley.

"This traffic, this flow, is really unprecedented," Qualia said.

The situation in El Paso is no different, where agents are taking in far more people on a daily basis than tents there can house.

Customs and Border Protection has also scheduled daily flights out of the Rio Grande Valley at least through Tuesday for some migrants, and they may continue depending on demand.

It's rare for the Border Patrol to use aircraft to send migrants to another location within the U.S. for processing. At a cost of about $6,000 each, the flights are managed by U.S. Immigration and Customs Enforcement, which has a network of contracted planes it uses to deport people.

The flights aim to ensure adults don't slip through the cracks as agents scramble to process the increasing number of families crossing the border.

Single adults are the only demographic to whom immigration authorities can currently apply a consequence, Border Patrol Chief Carla Provost told Congress last week.

"My greatest concern is that we will no longer be able to deliver consequences and we will lose control of the border," she said.

The bus to Denver cost New Mexico about $4,000. The governor's office hopes to minimize taxpayer expenses by soliciting charitable donations for additional trips.

The office announced in late April that it would make dormitories available to asylum seekers at the state fairgrounds in Albuquerque, though no one has stayed there so far. Prior bookings for a film production means the dormitories won't be available for migrants until later this month.

Santa Fe, just north of Albuquerque on the way to Denver, has declined suggestions that it provide shelter to migrants, but charitable fundraising efforts are underway.

Three Christian congregations in Denver volunteered to provide shelter to a busload of migrants.

The city had no role in coordinating or funding the effort, but politicians were praising the community for its eagerness to help.

"Denver will always be a welcoming place for people seeking refuge," Mayor Michael Hancock said.

New York also is seeing a significant increase in the number of unaccompanied minors who arrive and stay temporarily before being released to sponsors elsewhere.

Since October, more than 3,000 unaccompanied migrant children have been served by Catholic Charities in New York, a number that's likely to hit about 8,000 by this coming October, said Mario Russell, the group's director of Immigration and Refugee Services.

Some shelters have expanded their capacity to take more children, he said.

"That's a challenge and that is stretching our resources," Russell said. "We are working every day to expand our legal capacity, our physical capacity to do this and obviously it is something that is developing day by day."

Associated Press writers Colleen Long, Nomaan Merchant, Colleen Slevin and Claudia Torrens contributed to this report.