With border authorities in South Texas overwhelmed by a surge of young illegal migrants traveling by themselves, the Department of Homeland Security declared a crisis this week and moved to set up an emergency shelter for the youths at an Air Force base in San Antonio, officials said Friday.

After seeing children packed in a Border Patrol station in McAllen, Tex., during a visit last Sunday, Homeland Security Secretary Jeh Johnson on Monday declared “a level-four condition of readiness” in the Rio Grande Valley. The alert was an official recognition that federal agencies overseeing borders, immigration enforcement and child welfare had been outstripped by a sudden increase in unaccompanied minors in recent weeks.

On Sunday, Department of Health and Human Services officials will open a shelter for up to 1,000 minors at Lackland Air Force Base in Texas, authorities said, and will begin transferring youths there by land and air. The level-four alert is the highest for agencies handling children crossing the border illegally, and allows Homeland Security officials to call on emergency resources from other agencies, officials said.