WASHINGTON (Reuters) - The U.S. Navy is drafting plans to house up to 25,000 immigrants on its bases and other facilities, at an estimated cost of about $233 million over six months, as the Trump administration seeks to ease a mounting crisis on the Mexican border, a U.S. official said on Friday.

The Navy has not so far been asked to provide accommodation for migrants who have entered the United States illegally. The official, who asked not to be named, stressed that the draft memo, which looks at setting up housing on Navy airfields in Alabama, was for planning purposes only.

The news was first reported by Time Magazine.

President Donald Trump is facing a public outcry over his policy of separating children from their migrant parents, with lawmakers struggling to get immigration legislation passed.

The memo gives an early indication of how much Trump’s plan to house thousands of immigrants while they await trial under his “zero-tolerance” policy could cost.

The facilities, which could include tents, are described as “temporary and austere” in the memo, according to the official. It suggests the facilities be built for between six months and one year.

The official said the draft document, drawn up by Phyllis Bayer, the Assistant Secretary of the Navy for Energy, Installations and Environment, also says that a Navy base in California could house up to a further 47,000 people.

Asked about the plan, Pentagon spokesman Colonel Robert Manning said in a statement that the Department of Defense was “conducting prudent planning” and was looking nationwide at its installations in case it was asked for assistance in housing illegal immigrants.

The U.S. military said on Thursday it had been asked by the government to get ready to house up to 20,000 immigrant children.

It said the government had already assessed three military bases in Texas and would review another in Arkansas.

(This story corrects paragraph 4 to “migrant parents” instead of “migrant children”.)