The Trump administration is making preparations to warehouse migrant children on military bases, according to Defense Department communications, the latest sign the government is moving forward with plans to split up families who cross the border illegally.

According to an email notification sent to Pentagon staffers, the Department of Health and Human Services (HHS) will make site visits at four military installations in Texas and Arkansas during the next two weeks to evaluate their suitability for child shelters.

The bases would be used to hold minors under age 18 who arrive at the border without an adult relative or after the government has separated them from their parents. HHS is the government agency responsible for providing minors with foster care until another adult relative can assume custody.

The email characterised the site visits as a preliminary assessment. "No decisions have been made at this time," it states.

An official at HHS confirmed the military site visits, speaking on condition of anonymity because the plans are not yet public. The official said HHS currently has the bed space to hold 10,571 children.

In a statement, the agency's Administration for Children and Families said its programmes required "routinely evaluating the needs and capacity of an existing network of approximately 100 shelters in 14 states".

"Additional properties with existing infrastructure are routinely being identified and evaluated by federal agencies as potential locations for temporary sheltering," the statement said.

Those facilities are at 91 percent capacity, an HHS official said, and the Trump administration's crackdown plans could push thousands more children into government care.The official said DHS has not provided projections for how many additional children to expect.

Trump officials say they are moving forcefully to halt a sharp increase in the number of families crossing the border illegally this spring, many of whom are Central Americans seeking asylum. US border agents arrested more than 100,000 illegal border-crossers in March and April, the highest monthly totals since Donald Trump took office.

Trump has seethed at the increase, singling out Homeland Security Secretary Kirstjen Nielsen for blame. He has ordered her to "close" the border and cut off the migration flows, which typically increase in spring with seasonal demand for rural labour.

Ms Nielsen and Attorney General Jeff Sessions say the government will take the extraordinary measure of filing criminal charges against anyone who crosses the border illegally, including parents travelling with their children. In most cases, that means adults will be held at immigration jails awaiting court dates while their children are sent to foster care.

"If you're smuggling a child, then we're going to prosecute you, and that child will be separated from you, probably, as required by law," Mr Sessions said in a speech last week.

"If you don't want your child separated, then don't bring them across the border illegally" he added. "It's not our fault that somebody does that."

Children held in HHS custody spend an average of 45 days in the government's care, the HHS official said, and they are provided with educational and recreational opportunities. The agency conducts background checks on potential sponsors for the minors, and in 85 percent of cases the children are released to a parent or other adult relative already present in the United States, the official said.

Homeland Security officials have struggled for years to manage the demographic shift in the population of migrants arrested at the border, where single men from Mexico were once the overwhelming majority of those taken into custody.

Families with children and unaccompanied minors were 10 percent of illegal border crossers five years ago, Ms Nielsen told senators in testimony Tuesday. Today they account for 40 percent of those detained by US border agents, she said.

The use of military bases to hold migrant children is not without precedent. At the peak of the 2014 child migration crisis, the Obama administration used bases in Oklahoma, Texas and California to house more than 7,000 children over a period of several months.

Critics of family separation denounce the practice as heartless, saying it inflicts additional trauma on families fleeing for their lives from Central America's bloody gang wars.

Mr Trump last month ordered the Pentagon to help Homeland Security officials cope with the surge in illegal crossings, including the mobilisation of up to 4,000 National Guard troops. Military personnel have been deployed in a supporting role, and are not allowed to arrest migrants.

It's not clear whether the troops could be assigned to the bases where children will be held. Three of the bases are in Texas - the Army's Fort Bliss, Goodfellow Air Force Base and Dyess Air Force Base. Little Rock Air Force Base in Arkansas also will be evaluated, according to the Pentagon communications and HHS.

A Pentagon spokesman, Lt Col Jamie Davis, said the Defense Department had not yet received a formal request from HHS. When the military has loaned space at its facilities in the past, HHS has reimbursed the Defense Department and the military has had little to do with the operation.