The federal government is opening a new mass facility to hold migrant children in Texas and considering detaining hundreds more youths on three military bases around the country, adding up to 3,000 new beds to the already overtaxed system.

The new emergency facility in Carrizo Springs, Texas, will hold as many as 1,600 teens in a complex that once housed oil field workers on government-leased land near the border, said Mark Weber, a spokesman for Office of Refugee Resettlement.

The agency is also weighing using Army and Air Force bases in Georgia, Montana and Oklahoma to house an additional 1,...