The Department of Health and Human Services (HHS) made the request to the Department of Defense (DOD), and Congress has been notified, the official said.
The story was first reported by The Washington Post.
Last month, ABC News reported that HHS officials were touring four U.S. military bases to see if they could be used to house migrants in the event that other facilities reached capacity. Those bases were Dyess Air Force Base in Abilene, Texas, Goodfellow Air Force Base in San Angelo, Texas, Fort Bliss in El Paso, Texas, and Little Rock Air Force Base in Little Rock, Ark.
While officials have completed their tours of those installations, no final determination has been made as to where the unaccompanied migrant children would be located.
On Wednesday, Defense Secretary James Mattis told reporters that DOD would support the Department of Homeland Security (DHS) if requested.
"We have housed refugees, we have housed people thrown out of their homes by earthquakes and hurricanes. We do whatever is in the best interest of the country," Mattis said.
HHS has used facilities on U.S. military bases to house migrants in the past.
In 2014, the department used bases in Texas, Oklahoma, and California to house 7,000 unaccompanied migrant children after HHS facilities reached capacity.
Mattis has already signed a memo allowing up to 4,000 National Guard troops to assist DHS with the security of the U.S./Mexico border. About 2,000 troops, mostly from the National Guards of Arizona, New Mexico, and Texas, are serving there now -- but as support services to U.S. Customs and Border Patrol, not in a law enforcement capacity.
On Wednesday, President Trump signed an Executive Order, ending the forced separation of children, so that families who cross the border illegally will now be detained together.
Mattis told reporters on Wednesday that the withdrawal of Guard troops was not having an immediate impact on the border security mission.