Speaking at the White House Wednesday to follow up on President Donald Trump's surprise announcement that he would order the military to help secure the southern border, Homeland Security Secretary Kirstjen Nielsen refused to detail the size, scope or cost of a planned deployment of National Guard troops, but said it was hoped they could be deployed "immediately" – as early as Wednesday night.

Wednesday evening, a senior administration official said the president had signed a proclamation authorizing the National Guard to assist the border patrol.

Trump wants to send the U.S. military to protect the border with Mexico while the new border wall he wants is built and Nielsen also said the administration was "looking into options for the military to build a wall" on military installations on the border.

While the president could order active duty military personnel or federalized National Guard troops to assist with border security operations, they would not be allowed to carry out law enforcement duties like detaining migrants crossing the border illegally.

"The Department of Defense and the Department of Homeland Security have been directed to work together with our governors to deploy the National Guard to our southwest border, to assist the Border Patrol," Nielsen said.

She said the administration hopes the deployment of the National Guard will begin immediately and Trump officials are in touch and coordinating with governors in impacted states. She also said that the U.S. has also been in touch with Mexico about the decision and that Mexicans “understand and respect our national sovereignty.”

President George W. Bush in 2006 and President Barack Obama in 2010 also sent thousands of National Guard troops to assist with border operations, primarily in administrative and surveillance roles, in order to free up Customs & Border Protection personnel to focus on enforcement.

White House press secretary Sarah Sanders blamed Congressional inaction, explaining that the president is taking action within his power to secure the border.

“The president has been talking about it for years, since he started on the campaign trail,” Sanders said. “He wanted to work through Congress. He asked them to do their jobs. He asked them to pass legislation that actually would close loopholes that would secure our border."

“He is doing what he can to protect the people of this country and he'll continue to do that and look at different measures that he can do it,” she added.

Nielsen punted to the Pentagon when asked whether the military would cover any costs of the construction of the border wall, explaining that the president was referring to improvements to military infrastructure along the border.

“As part of what DOD does every day, in terms of force protection, we're looking into options for the military to build a wall on military installations on the border,” Neilsen said.

Nielsen said the administration considers any wall that has to be prepared or replaced would count as new wall.

“If there's a wall before that needs to be replaced, it's being replaced by a new wall. So this is the Trump border wall,” she said.

Nielsen admitted that the president has been "frustrated" over the issue, but said the option of deploying National Guard troops to the border has always been on the table.