At least 1,000 unaccompanied children who crossed illegally into the United States through Texas were being taken to a makeshift emergency shelter in Arizona over the weekend.
It was the latest effort by authorities scrambling to handle what has been described as a humanitarian crisis.
Although overall illegal immigration has declined in recent years, two waves - one of unaccompanied children, another of parents with children - have presented a challenge for officials who say they do not have the facilities in the southwestern US to detain these groups.
The presence of unaccompanied migrant children is not new, but the surge in recent months has overloaded Border Patrol stations and detention facilities, particularly in Texas. Most of the children come from Central America, a region long plagued with poverty but now having to grapple with escalating drug cartel and gang violence.
On Saturday alone, 367 children were taken from Texas to a processing centre run by US Customs and Border Protection in Nogales, Arizona, said Andrew Wilder, spokesman for state Governor Jan Brewer.
"We fully expect this crisis to continue because there is no solution to fix it," Wilder said.
Brewer blasted the transfers and, in a letter to President Barack Obama, complained that she learned of the operation through the media, not from his administration.
She has yet to hear back from Obama, Wilder said.
The unaccompanied children housed in Nogales are supposed to stay for up to 72 hours before they are sent to longer-term facilities at military installations in California, Texas and Oklahoma.
A shelter in San Antonio, Texas, opened two years ago to cope with an earlier surge of immigrant minors. The facility closed after two months as officials found ways to place youths more quickly. But two weeks ago, overwhelmed again by a new surge of unaccompanied minors, officials reopened the shelter.
It is already approaching its capacity of 1,200. Another shelter, capable of housing 600 youths, opened on Friday at Port Hueneme in California.
Immigrant advocates say they understand that the government is pressed to house young migrants, and that the shelters are stopgap measures. But they fear the youths may languish in the institutional settings.
The young migrants' ranks have tripled in five years, and could reach a new high of 60,000 this year - and more than double that next year. By then, the costs of shelters and resettlement could reach US$2.28 billion.
Obama has directed a Unified Coordination Group to address the growing numbers of unaccompanied young migrants, a trend administration officials characterised as an "urgent humanitarian situation".
On Friday, US Attorney General Eric Holder announced the start of a new effort, coordinated with the AmeriCorps community service programme, to provide about 100 lawyers and paralegals to immigrant children.