(Credit: OMAR TORRES/AFP/Getty Images)

President Obama today declared an "urgent humanitarian situation" along the southwest U.S. border after a sudden surge of hundreds of unaccompanied, undocumented children flowing into the country in the past month.

The declaration, made in a presidential memorandum, directs federal agencies to coordinate an emergency response to the situation and ensure the children receive proper medical care, housing and transportation.

The White House has appointed FEMA administrator Craig Fugate to lead the effort.

While the number of children crossing the border illegally has been steadily on the rise in recent years, it spiked in the past month - up 90 percent from this time last year, said White House domestic policy director Cecilia Munoz.

Officials said more of the children detained are girls, and more are younger than 13, compared with last year. Most crossed into the U.S. through the Rio Grande valley in Texas. They are predominantly from Central American countries where economic conditions, sustained violence and a desire to reunite with their parents in the U.S. have driven them northward, Munoz said.

Once they are detained along the border, Homeland Security officials are required under federal law to transfer the minors to the custody of Health and Human Services within 72 hours.

HHS says that for the past two weeks more than 1000 children have been housed at temporary facilities on Lackland Air Force Base near San Antonio, Texas. The Pentagon has approved the use of another military facility -Naval Base Ventura County in Oxnard, Calif. - to accommodate an additional 600 children, officials said. It is not yet operational. The military would have no role in caring for or handling the children aside from making facilities available, they said.

The Obama administration has asked Congress for an additional $1.4 billion to process and care for the influx of immigrant kids. The Office of Management and Budget anticipates the federal government will spend $2.28 billion this year handling the situation.

Officials said the average stay in an HHS facility is 30-45 days before the child is released to a parent, relative or other sponsor.

"The law does not allow expedited removal of children from non-contiguous countries," Munoz said. "Our primary mission….is to make sure they're returned to best" situation. The children remain in so-called "removal proceedings" even if they are released. "The immigration process does not stop when reunited with family members," she said.

Officials said they are working with the governments of Mexico, Guatemala and El Salvador to "spread the word" against travel to the U.S., including distribution of public service announcements raising awareness of the perils.

Department of Homeland Security officials said they would provide later today or tomorrow total figures for the number of immigrant children who have crossed the border in recent months, and are or have been in U.S. custody.