US Defence Secretary James Mattis has said military police at the border with Mexico will be unarmed and will not have the power to carry out arrests.
“They don’t have guns in their hands, there is no armed element going in,” Mr Mattis told reporters on Wednesday.
He said the military deployment along the Mexican border was to help protect border officials from possible threats.
A convoy of almost 3,000 Central American migrants has arrived at the Mexican border city of Tijuana.
They say they are fleeing persecution, poverty and violence in their home countries of Honduras, Guatemala, and El Salvador.
But President Donald Trump has described the caravan of migrants hoping to cross into the US as an “invasion”, and deployed about 5,800 troops to the southern border to “harden” it.