The Pentagon announced the deployment of 4,800 US troops on Monday to the border with Mexico, without specifying the cost of the process denounced by Democrats as a "political maneuver" for President Donald Trump.
About 1,100 troops were deployed in California, 1,100 in Arizona and 2,600 in Texas, on the eve of a crucial mid-term election that will determine whether Republicans retain control of Congress.
"The situation is still changing with the deployment of more units and soldiers in the region, and we expect the number of troops deployed today to reach 5200," said Colonel Bob Manning, a spokesman for the Defense Ministry.
Manning added that more than 7,000 regular troops are expected to "soon" provide support to the Department of Homeland Security.
Some 2,100 National Guard reserve soldiers have been deployed in the area a few months ago.
With the arrival of new numbers, the US military presence at the border with Mexico will rise to 9,000.
The deployment is aimed at preventing the arrival of a convoy of Central American immigrants heading for the United States, where they intend to seek asylum.
Asked about the cost of the operation, Manning said it had not yet been identified by the Pentagon's financial services.
"The ministry will absorb the cost, but I do not have a figure for you," he said.
Manning pointed out that the Department of Homeland Security had asked that regular soldiers, not reservists - who usually take part in operations on US soil - be involved in the process.
This suggests that the White House is trying to circumvent the opposition of some governors who have to agree to deploy some of the National Guard troops under their mandate.
"There is no plan for them to have direct contact with migrants or demonstrators," Manning said.
Trump, who warned of the "invasion" of thousands of immigrants now in Mexico, announced the deployment last week, saying up to 15,000 troops could be involved.
