Some 1,100 of the troops are in California, another 1,100 in Arizona and 2,600 in Texas on the eve of crucial midterm elections that will determine whether Trump's Republicans keep their hold on power in Congress.

"This continues to be a dynamic situation with more units and personnel deploying to the operating area and we expect to reach 5,200 deployed personnel as early as today," said Colonel Bob Manning, a Defense Department spokesman.

Manning said more than 7,000 active duty troops were expected to "soon" be supporting the Department of Homeland Security.

Some 2,100 National Guard reservists have already been active in the area for several months, so the arrival of the latest troops will bring the total US military presence to 9,000.

Operation Faithful Patriot aims to block the huge groups of Central American migrants marching on foot toward the United States, where they plan to demand asylum.

Asked about the cost of the operation, Manning said it had yet to be determined by the Pentagon's financial services.

"The department will absorb the cost but I just don't have a number for you," he said.

Manning noted that DHS had requested that active duty troops, rather than reservists -- who usually partake in operations on US soil -- for this operation.

That's a sign that the White House was trying to circumvent the opposition of some governors, who need to approve the deployment of their state's National Guard troops.

"There is no plan for them to come in direct contact with migrants or protesters," Manning said.

Trump, who has warned of an "invasion" of the thousands of migrants now in Mexico, announced the military deployment last week, saying that up to 15,000 troops could serve in the operation.