The Trump administration has proposed spending $US18 billion ($A25 billion) over 10 years to significantly extend the border wall with Mexico, providing one of its most detailed blueprints of how the president hopes to carry out a signature campaign pledge.
The proposal by Customs and Border Protection calls for 505 kilometres of additional barrier by September 2027, bringing total coverage to 1,552 kilometres, or nearly half the border, according to a US official with direct knowledge of the matter.
It also calls for 651 kilometres of replacement or secondary fencing, said the official, who spoke to The Associated Press on the condition of anonymity because the plan has not been made public.
Trump has promised "a big, beautiful wall" with Mexico as a centrepiece of his presidency but offered few details of where it would be built, when and at what cost. His administration asked for $US1.6 billion this year to build or replace 118 kilometres of fencing in Texas and California, and officials have said they also will seek $US1.6 billion next year.
Mexico has steadfastly rejected Trump's demand that it pay for the wall and few doubt that US taxpayers will foot the bill if the wall is built.
The Customs and Border Protection document calls for a total of $US33 billion in new border spending, including $US18 billion for the wall, $US5.7 billion for technology gear, $US1 billion for road construction and maintenance and $8.5 billion for 5,000 new Border Patrol agents, 2,500 border inspectors and other personnel, the US official said.
The document doesn't specify where the extended wall should be built.
It 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.
Trump said last year that he was ending DACA but gave Congress until March to deliver a legislative fix.
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.
