A week after announcing it would divert $3.6 billion in funds for the construction of an anti-migrant wall on the frontier, the Pentagon said it had also approved a request from the Department of Homeland Security to maintain a troop presence as well.

"DoD will provide up to 5,500 personnel to provide infrastructure support; operational support; detection and monitoring support; and air support," said Pentagon spokesman Lt. Col. Chris Mitchell.

The deployment, which began nearly a year ago, was approved by Defense Secretary Mark Esper for the coming fiscal year beginning October 1.

Currently around 2,900 soldiers and 2,000 reservists are operating at the border.

Mitchell said using military personnel to bolster a civilian operation to detain tens of thousands of people attempting to enter the country illegally each month had no significant impact on the US military's combat readiness.

"These missions can be supported with manageable impacts to readiness, and are contingent on the availability of funds and the continued statutory authority to provide such support," he said.

President Donald Trump has made halting undocumented immigration a key thrust of his administration.

The diversion of billions of dollars of Pentagon funds -- already allocated for some 127 projects -- to border wall construction was justified under a controversial emergency declaration by Trump.

The Trump administration declared a national emergency in February of this year after Congress repeatedly denied the president funding for the construction of a border wall that he promised Mexico would pay for.

On Tuesday, senior Democratic Senator Chuck Schumer announced that the party would soon introduce a resolution to terminate the emergency declaration, which would block the use of the funds on border wall projects.

"The president's national emergency declaration was, and is, an outrageous power grab by a president who refuses to respect the constitutional separation of powers," Schumer said.