President Obama on Monday described a surge in unaccompanied immigrant children caught trying to cross the Mexican border as an "urgent humanitarian situation," as the White House asked Congress for an extra $1.4 billion in federal money to cope. Obama said the U.S. will temporarily house the children at two military bases.

Obama appointed the head of the Federal Emergency Management Agency, Craig Fugate, to be in charge of the situation.

In its new estimates, the government said as many as 60,000 children, mostly from Central America, could be caught this year trying to cross the Mexican border illegally, costing the U.S. more than $2.28 billion to house, feed and transport the children to shelters or reunite them with relatives already living in the United States. The new estimate is about $1.4 billion more than the government asked for in Obama's budget request sent to Congress earlier this year.

Obama described the growing humanitarian issue at the border in a presidential memorandum Monday that outlined a government-wide response led by Fugate.

Obama's director of domestic policy, Cecilia Munoz, said the number of children traveling alone has been on the rise since 2009, but the increase was larger than last year. Munoz said the group also now includes more girls and larger numbers of children younger than 13.

"All of these things are contributing to the sense of urgency," Munoz said. "These are children who have gone through a harrowing experience alone. We're providing for their proper care."

The growth has surpassed the system's capacity to process and house the children. Last month, the federal government opened an emergency operations center at a border headquarters in South Texas to help coordinate the efforts and the Office of Refugee Resettlement, a division of the Health and Human Services Department, turned to the Defense Department for the second time since 2012 to help house children in barracks at Lackland Air Force Base near San Antonio.

Mark Greenberg, an assistant secretary at the Health and Human Services Department, said about 1,000 children were being housed at the Texas base and as many as 600 others could soon be housed at a U.S. Navy base in Southern California.

The number of children found trying to cross the Mexican border without parents has skyrocketed in recent years. Between 2008 and 2011, the number of children landing in the custody of Refugee Resettlement fluctuated between 6,000 and 7,500 per year. In 2012 border agents apprehended 13,625 unaccompanied children and that number surged even more -- to 24,668 -- last year. The total is expected to exceed 60,000 this year.

More than 90 percent of those sheltered by the government are from Honduras, Guatemala and El Salvador, many driven north by pervasive violence and poverty in their home countries. They are held in agency-contracted shelters while a search is conducted for family, a sponsor or a foster parent who can care for them through their immigration court hearings, where many will apply for asylum or other special protective status. Border Patrol agents have said that smugglers are increasingly notifying authorities once they get children across the Rio Grande so that they can be picked up.

Rampant crime and poverty across Central America and a desire to reunite with parents or other relatives are thought to be driving many of the young immigrants. Munoz said Monday the administration is aware of false rumors that have circulated that migrant children who get to this country would be automatically allowed to stay here or benefit from some future immigration reform legislation.

Migrant kids remain in removal proceedings even after they're reunited with their parents here, though many have been able to win permission from a judge to stay in the U.S.

The Office of Management and Budget said in a two-page letter to Sen. Barbara Mikulski, the chairwoman of the Senate Appropriations Committee, last month that the increase in children trying to cross the border alone could cost the government as much as $2.28 billion. The administration originally asked Congress for $868 million for the "Unaccompanied Alien Children" program run by Health and Human Services, the same amount Congress approved last year.

Brian Deese, deputy director of the budget office, said the Homeland Security Department would also need an extra $166 million to help pay overtime costs for Customs and Border Protection officers and agents, contract services for care of the children and transportation costs.

A House appropriations subcommittee voted last week to add $77 million to the original request. Deese sent the letter to Mikulski a day after the House subcommittee vote.