President Donald Trump will be signing a proclamation directing the Departments of Defence and Homeland Security to work together with governors to deploy the National Guard to the border with Mexico.
Homeland Security Secretary Kirstjen Nielsen made the announcement at the White House on Wednesday.
She said the president would sign the proclamation later in the day.
Ms Nielsen condemned the country’s current immigration laws and calling on Congress to make changes.
She said the current system “rewards bad behaviour” and “it’s time to act”.
She says details are still being worked out, but National Guard personnel are expected to assist with US Customs and Border Protection’s mission.
Mr Trump had announced on Tuesday that he plans to deploy the military to the border with Mexico until his long-promised wall is built.
The president had earlier Wednesday tweeted pledging “strong action today”, a day after he said he wants to use the military to secure the US-Mexico border until his “big, beautiful wall” is erected.
In an early-morning tweet, the president said “Our Border Laws are very weak” and claimed Democrats “stand in our way” of new laws.
He added: “We will be taking strong action today.”