The National Guard will deploy up to 4,000 troops to the U.S.-Mexico border beginning Friday night, according to a newly signed memo from Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, after President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE called for troops to shore up the area until his border wall is built.

The memo authorizes the use of Title 32 and Defense Department dollars for up to 4,000 National Guard personnel to support the Department of Homeland Security’s “southern border security mission while under the command and control of their respective governors through September 30, 2018.”

It also states that troops "will not perform law enforcement activities or interact with migrants or other persons detained by DHS personnel” unless Mattis approves it.

ADVERTISEMENT

Troops will be armed only in “circumstances that might require self-defense," according to the memo.

National Guard Bureau Chief Gen. Joseph Lengyel wrote on Twitter that up to 500 guardsmen are now moving to the border along with vehicles, helicopters and other equipment.

“Always Ready, Always There! Moving up to 500 #NationalGuard troops immediately on the SW border security mission. Vehicles, equipment & helicopters on the way tonight,” Lengyel wrote.

The memo does not say how much the mission will cost.

Trump earlier this week signed a presidential memorandum directing Mattis to support DHS in securing the southern border to address a “drastic surge of illegal activity on the southern border.”

Trump then said Thursday that he wants to send between 2,000 and 4,000 guardsmen to the border to keep at bay illegal activities such as drug smuggling until his proposed border wall is built.

Arizona and Texas have already announced plans to send troops to the border starting this weekend.

Arizona Gov. Doug Ducey (R) announced Friday afternoon on Twitter that he would be sending around 150 Guard troops.

Army National Guard Brig. Gen. Tracy Norris, meanwhile, told reporters about 250 troops will be sent by Monday to Texas’s border with Mexico.

"This mobilization supports the priorities of the governor and the president in securing our borders,” Norris said.

Norris added that the Texas deployment began Friday and more troops will be sent “once mission requirements and locations are finalized.”

“As early as tomorrow, notifications will go out to soldiers who will be called up as part of the follow-on phase,” telling troops they need to report as early as next week, Norris said.

Former Texas Gov. Rick Perry Rick PerryOVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project Senate votes to confirm Energy's No. 2 official MORE (R) sent the Guard to the border to help secure it in 2014, and 100 soldiers remain there from that effort.

The deployment of a potential 4,000 guardsmen to the border is lower than the 6,000 Guard troops former President George W. Bush sent in 2006 but higher than the 1,200 former President Obama sent in 2010. Both used Title 32.

Bush’s deployment cost $415 million from the Air Force and Navy accounts at the time, but DHS and Pentagon officials have been unable to answer how much money it will cost this time around and where the dollars will come from.

The Pentagon’s chief spokeswoman Dana White on Thursday would not confirm that the funds would not come from the Pentagon’s operation and maintenance (O&M) account, which is used for military readiness.

House Armed Services Committee Chairman Mac Thornberry William (Mac) McClellan ThornberryTrump payroll-tax deferral for federal workers sparks backlash Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq Top Armed Services Republican 'dismayed' at Trump comments on military leaders MORE (R-Texas) earlier this week stressed that the Defense Department needs all of its allocated funding for military readiness.

Asked Friday if O&M funds will be used to pay for the border deployment, Mattis only replied that the Pentagon is "looking at how we can best provide the support” to DHS.

"We'll figure it out. It will be consistent with law and the spirit of Congress. No problem."