The move took to $9.9 billion the total the Pentagon has been forced by President Donald Trump to reallocate to the barrier, after Congress repeatedly blocked funding.

The president declared a national emergency last year to be able to draw federal budget funds already allocated to other needs for the wall, which aims to deter migrants from entering the country illegally.

In January an appeals court backed Trump's diversion of already-purposed federal budget funds to the project, removing a stay on spending set by a lower court after opponents filed suit.

The Pentagon told Congress it was transferring the $3.8 billion to "counter-drug activities" on the border in support of the Department of Homeland Security.

The money 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 turned back 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.