President Donald Trump will ask the US Congress for an additional $US8.6 billion ($A12.2 billion) to help pay for the wall he promised to build on the southern border with Mexico, officials say.
The demand is more than six times what congress allocated for border projects in each of the past two fiscal years and 6 per cent more than Trump has corralled by invoking emergency powers this year.
Democrats, who oppose the wall as unnecessary and immoral, control the US House of Representatives, making it unlikely the Republican president's request will win congressional passage. Republicans control the Senate.
The proposal comes on the heels of a bruising battle with congress over wall funding that resulted in a five-week partial federal government shutdown that ended in January, and could touch off a sequel.
Asked on Fox News Sunday about the new funding request and if there would be another budget fight over Trump's wall, White House economic adviser Larry Kudlow said, "I suppose there will be.... He's going to stay with his wall and he's going to stay with the border security theme. I think it's essential."
Broadly speaking on the budget, Kudlow told Fox, "The president is proposing roughly a 5 per cent across-the-board reduction in domestic spending accounts."
Regardless of whether congress passes it, the budget request could help Trump frame his argument on border security as the 2020 presidential race begins to take shape, with the president seeking re-election.
"Build the wall" was one of his signature campaign pledges in his first run for office in 2016. "Finish the wall" is already a feature of his re-election campaign, a rallying cry plastered across banners and signs at his campaign rallies.
"It gives the president the ability to say he has fulfilled his commitment to gain operational control of the southwest border," an administration official said of the budget request.
"We have provided the course of action, the strategy and the request to finish the job. It's a question of, will congress allow us to finish the job," a second administration official said.
Funding legislation needs to be passed before October 1 - the start of the 2020 federal fiscal year - or the government could shut down again.
Trump's wall request is based off a 2017 plan put forward by Customs and Border Protection officials to build or replace 1162km of barrier along the border, which in total is estimated to cost about $US18 billion.
So far, only 179km have been built or are under way, officials said. In fiscal 2017, $US341 million in funding was allocated for 64km of wall, and in 2018, another $US1.375 billion was directed to 132km.
For fiscal 2019, Trump demanded $US5.7 billion in wall funds, but congress appropriated only $US1.375 billion for border fencing projects.
Following the rejection of his wall funding demand, Trump declared the border was a national emergency and redirected $US601 million in Treasury Department forfeiture funds, $US2.5 billion in Defense Department drug interdiction funds and $US 3.6 billion from a military construction budget, for total spending of $US8.1 billion for the wall.
Trump faces both political and court battles to free up the money he wants for the current fiscal year.
Many lawmakers accused Trump of overstepping his constitutional powers by declaring an emergency to free up the funds. The House has already voted to revoke the emergency, and the Senate is likely to do the same this week.
Trump is expected to veto the resolution.
