Defence Secretary James Mattis is expected to deploy around 800 troops to the US-Mexico border to stop a migrant caravan, US media say.
The soldiers would assist with fencing and support at key points along the southern border, administration officials tell CNN.
The caravan of thousands of Central American migrants is making its way toward the US through Mexico.
President Donald Trump said on Thursday he was “bringing out the military”.
“I am bringing out the military for this National Emergency,” Mr Trump said of the migrant caravan in a tweet, hinting of an upcoming deployment. “They will be stopped!”
Secretary Mattis’ order is in response to a request from President Trump, according to the Associated Press.
Mr Trump first threatened such action last week, saying he would use the military to completely close the US-Mexico border. He also threatened cutting aid to Guatemala, El Salvador and Honduras.
It remains unclear whether these 800 troops will be National Guard or active military forces, but the Washington Post reported an official saying some active Army personnel would be deployed.
There are already thousands of National Guard troops supporting border officials – 2,100 unarmed troops are at the border presently – but 4,000 have been approved for deployment by the Pentagon.