Pentagon officials said the money would be used to build about 177 miles (285 kilometers) of the concrete and steel barrier Trump wants to deter migrants from entering the country illegally.

That will bring to a hefty $9.9 billion the total amount the US military has been forced to divert to border wall construction over the past two years as Trump seeks to overcome Congress's repeated refusal to provide the funding.

The Pentagon called the money a contribution to the Department of Homeland Security's efforts to combat drug trafficking.

But Trump has made clear that the barrier targets migrants from the south.

The wall construction "will allow DHS to fulfill the president's border security policy promise," said Deputy Assistant Secretary of Defense Robert Salesses.

"The Corps of Engineers, under the direction of the Secretary of the Army, is ready to contract and construct the border barrier."

Trump declared a national emergency last year to overcome Congress's resistance and draw federal budget funds already allocated to other needs for the wall.

In a new White House order Thursday, that emergency was extended through February 2021, despite a sharp decline in migrants crossing the border from Mexico illegally.

"The ongoing border security and humanitarian crisis at the southern border of the United States continues to threaten our national security," the order said.

"Further action is needed to address the humanitarian crisis and to control unlawful migration and the flow of narcotics and criminals across the southern border of the United States."

Last year opponents of the wall sued to block the use of military funds, and a judge placed a stay on any spending pending a hearing on the case.

But in January an appeals court lifted the stay and allowed the Trump administration to go ahead and deploy the Pentagon funds for the project.

- Election promise -

The $3.8 billion announced Thursday was to be culled from various programs for buying tactical vehicles for the Army National Guard, vessels for the Navy, and combat and transport aircraft for the Navy and Air Force.

Trump promised to build the wall along most of the 2,000-mile (3,200-kilometer) US-Mexico border -- paid for by Mexico -- during his 2016 presidential campaign.

After he was elected however he sought some $25 billion from Congress but was repeatedly rebuffed amid opposition to his harsh stance against undocumented migrants already living in the country.

Meanwhile the number of illegal border-crossers detained soared as hundreds of thousands of migrants poured into the US, mostly from Central America, seeking asylum.

A crackdown and changes in policy that made it far more difficult stay in the United States after submitting an asylum request has seen border apprehension numbers plunge in recent months.

The figure stood at fewer than 37,000 last month, from more than 58,000 a year earlier.

Salesses told reporters it was possible that the Pentagon could also be asked to divert money from its own construction projects to the border this year.

"I am aware of ongoing discussions, but no decisions have been made," he said.