The Department of Health and Human Services (HHS), "requested DoD support to identify space to house up to 5,000 unaccompanied alien children on DoD installations, if needed, through September 30, 2019," said Pentagon spokesperson Lieutenant Colonel Jamie Davis.

"DoD will work with the military services to identify potential locations for such support, and will work with HHS to assess any DoD facilities or suitable DoD land for potential use to provide temporary shelter for unaccompanied alien children," he said.

The request for beds is preventative -- they will not definitely be used.

But it comes amid a surge in the number of families and unaccompanied children crossing the US border from Mexico illegally, most of them fleeing poverty and violence in Central America. Most request asylum.

According to the Customs and Border Protection Agency (CBP), the number of minors apprehended after entering the country illegally without family in February hit 6,825, up from 5,119 in January and 4,968 in October.

CBP hands the children over to HHS's Office of Refugee Resettlement, which seeks to place them with relatives or other families in the United States while their requests to remain in the country are processed, which can take two years.

At times last year they had more than 15,000 such children in their care at one time.

CBP Commissioner Kevin McAleenan said on Tuesday that illegal immigration in February surged to the highest level in years, with 76,000 people stopped or apprehended, and was straining government facilities.

"We are currently facing a humanitarian and national security crisis along our southwest border," he said.

"The vast increases in families and children coming across our border, in larger groups and in more remote areas, presents a unique challenge to our operations and facilities, and those of our partners."

On Wednesday Homeland Security Secretary Kirstjen Nielsen said they were expecting the numbers to grow in the coming months.

Nielsen though said in many cases the children were being used as "pawns" to get into our country, and even "recycled" by smuggling rings to help multiple groups cross the border and get a foothold on US soil.

She said she sympathized with the migrants but insisted that, as Trump formally declared last month, "It is an emergency."