The Trump administration has asked Congress to set aside $18 billion over the next decade to install or replace more than 700 miles of the barrier along the U.S.-Mexico border, The Wall Street Journal reported Friday.

The proposal by U.S. Customs and Border Protection would extend the barrier's coverage to 970 miles, nearly half the distance of the southern border. It includes 316 miles of new construction and 407 miles of replacement or secondary fencing, a U.S. official with direct knowledge of the matter told the Associated Press.

The request is one of the most detailed insights into how the administration plans to carry out Trump's signature campaign promise.

Trump has repeatedly promised to construct "a big, beautiful wall," but offered few details of where it would be built, when and at what cost. His administration asked for $1.6 billion this year to build or replace 74 miles of fencing in Texas and California, and officials have said they also will seek $1.6 billion next year.

The 10-year plan resulted from discussions with senators who asked the agency what it would take to secure the border, the official said.

CONTRACTOR HOPES FOR A PIECE OF TRUMP'S BORDER WALL -- NO MATTER WHO PAYS FOR IT

The plan also comes as the administration intensifies negotiations in Congress on a package that may include granting legal status to about 800,000 people who were temporarily shielded from deportation under an Obama-era program, Deferred Action for Childhood Arrivals, or DACA.

Trump said last year that he was ending DACA but gave Congress until early March to deliver a legislative fix.

House Minority Leader Nancy Pelosi, D-Calif., described the CBP proposal as "alarming" in a letter to fellow House Democrats Friday morning. Senate Minority Whip Dick Durbin, D-Ill., told The Wall Street Journal that the plan threatened to undermine negotiations on a spending bill needed to keep the government open later this month.

"It’s outrageous that the White House would undercut months of bipartisan efforts by again trying to put its entire wish list of hard-line anti-immigrant bills — plus an additional $18 billion in wall funding—on the backs of these young people," Durbin said.

The plan on border security came in response to a request by Sen. Jeff Flake, R-Ariz., according to Flake spokesman Jason Samuels.

An administration official confirmed the document was prepared at the request of congressional negotiators and said funding for the wall and other security measures must be part of any legislative package on immigration. The official spoke on condition of anonymity because the plan has not been made public.

Homeland Security Secretary Kirstjen Nielsen told The Associated Press on Tuesday that the wall would be "first and foremost" in any package that includes new protections for DACA recipients. She also said the administration wanted to close "loopholes" on issues that include handling asylum claims and local police working with immigration authorities.

Nielsen called the $3.2 billion requests for fencing during the administration's first two years a down payment.

"This is not going to get us the whole wall we need, but it's a start," she said.

Mexico has steadfastly rejected Trump's demand that it pay for the wall and few doubt that U.S. taxpayers will foot the bill if the wall is built.

The CBP document includes $5.7 billion for towers, surveillance equipment and other technology; $1 billion over five years for road construction and maintenance; and $8.5 billion over seven years for 5,000 new Border Patrol agents, 2,500 border inspectors and other personnel, the U.S. official said.

The document doesn't specify where the extended wall should be built.

Fox News' Chad Pergram and The Associated Press contributed to this report.

Click for more from The Wall Street Journal.