Secretary of Defense Jim Mattis and Homeland Security Secretary Kirsten Nielsen, third from the right, visit Base Camp Donna, in Donna, Texas, Nov. 14, 2018. Mattis says the White House has given him the authority to use military troops to protect Customs and Border Protection personnel at the southwest border. This could, under certain circumstances, mean directing troops to temporarily detain migrants in the event of disorder or violence against border patrol agents. Credit: Joel Martinez | The Monitor via AP

The Trump administration has given U.S. troops along the southern border permission to use force in protection of federal Border Patrol agents, but there has been no decision to alter the military’s mission there, Defense Secretary Jim Mattis said Wednesday.

Mattis, speaking to reporters at the Pentagon, said he was reviewing an order the White House issued late Tuesday for service members deployed in support of Customs and Border Protection personnel in Texas, Arizona and California.

According to the order, signed by White House Chief of Staff John Kelly, military personnel may use “a show or use of force (including lethal force, where necessary), crowd control, temporary detention and cursory search” to protect CBP.

“It is not an unreasonable concern on the part of the president that we may have to back up Border Patrol,” Mattis said, citing skirmishes between Mexican authorities and migrants on that country’s border with Central America earlier this fall.

However, he said he had not altered the orders of the approximately 5,800 active-duty troops arrayed across the southern border, most of whom are not armed and have been performing functions such as stringing concertina wire, building barriers and transporting Border Patrol agents.

Mattis has repeatedly defended President Donald Trump’s decision to use active-duty military personnel to harden the southern border, a move that Democrats characterized as a political stunt ahead of midterm elections earlier this month. The president remains focused on the caravans of mostly Central American migrants who are approaching the southern border from Mexico with hopes of seeking asylum in the United States. Trump has labeled them a threat to national security.

Critics have raised questions about the need to use active-duty troops for a mission already involving Border Patrol agents and National Guard personnel, citing concerns the deployment could violate the Posse Comitatus Act, a law limiting military activities within the United States.

Hogan Gidley, the deputy White House press secretary, said the new order “ensures the Department of Defense can step in to protect those who protect us.”

“The brave men and women at Customs [and] Border Protection willingly put themselves in extremely dangerous situations every day to protect Americans and their families,” Gidley said in a statement.

Mattis said the Department of Homeland Security, which oversees Customs and Border Protection activities, had not asked for troops to use force to protect its personnel and that he would assess any such request if it were made. Most active-duty troops along the border do not carry firearms, Mattis said. If they were tasked with protecting CBP, they would probably use shields and batons, he said.

Mattis and other Pentagon officials have sought to depict the role of U.S. troops as secondary to CBP, and they minimized the likelihood military personnel would need to use force in the way they do when deployed overseas.

“We are not doing law enforcement. We do not have arrest authority,” Mattis said. He said troops might take custody of individuals for a short time if someone attacked a CBP officer and troops were asked to respond. “This is minutes, not even hours,” he said.

Mattis dismissed concerns about potential legal violations. “Relax. Don’t worry about it, OK?” he said.

Some civil liberties and human rights groups, though, criticized the administration’s latest move. “This legally dubious ‘cabinet order’ creates confusion, undermines morale, and may very well lead to violence,” Michael Breen, president of Human Rights First, said in a statement.

Mattis said it wasn’t yet clear whether the border mission would be extended beyond its current end date of Dec. 15 or if the new White House order would necessitate sending additional troops to the border. Earlier this week, the Army general leading the border mission signaled his interest in beginning to wind it down.

Also on Wednesday, Mattis said he expected the active-duty deployment to cost more than the $72 million estimate the Pentagon released the day before. That’s in addition to the $103 million the Pentagon spent to deploy the National Guard to the border from April 10 to Sept. 30, and the $308 million the Pentagon expects to spend on the Guard deployment from October through the end of September next year.

All told, the cost of both the active-duty and National Guard deployments looks set to total more than $483 million by the end of this fiscal year in late September 2019.

Washington Post writers Nick Miroff and Josh Dawsey contributed to this report.