Members of the U.S. military place concertina wire along the border fence near the California-Mexico border on Nov. 29. (Carolyn Van Houten/The Washington Post)

The Pentagon is preparing to dispatch several thousand more U.S. troops to the southern border as the military shifts its operations there from improving security at ports of entry to the vast areas between them that are less controlled, defense officials said Tuesday.

Acting defense secretary Patrick M. Shanahan told reporters at the Pentagon that the evolving mission will focus heavily on watching the border for potential threats. It will mark the latest shift in how active-duty troops are used on the border, and nudge the number from about 2,300 active-duty service members closer to a high-water mark of about 5,900 that were involved in November. An additional 2,300 national guardsmen also are deployed to the border mission.

"It's really around this mission of monitoring, surveilling and detection," Shanahan said of the new deployments.

A Pentagon spokesman, Army Lt. Col. Jamie Davis, said the military is still determining which units will be involved.

“We will provide more clarity on the numbers when we have it,” he said.

On Capitol Hill, defense officials said that a Department of Homeland Security request approved this month by Shanahan will have U.S. troops operating mobile surveillance cameras across the border. U.S. troops also will install more concertina wire, an ongoing mission that will shift from ports of entry at the border to sections between them in Arizona and California.

The camera support will continue through the end of September, said John Rood, the undersecretary of defense for policy, testifying before the House Armed Services Committee. An additional 150 miles of concertina wire will be emplaced by the end of March, he said.

The operations are a continuation of a mission that began at the Trump administration’s direction days before the midterm elections, prompting allegations from Democrats and some retired military officers that President Trump was thrusting the military into partisan politics. But there was little mention of that at the hearing Wednesday, as lawmakers from both major political parties focused on the scope and future of the mission.

Rep. Adam Smith (D.-Wash.), the committee chairman, said the use of active-duty troops at the border “is a little different” and had prompted him to call the hearing. He asked the witnesses why the deployment was necessary, adding: “I don’t see it.”

Defense officials said the troops helped bolster the Department of Homeland Security’s abilities at the border at a time when it was not clear how and when a caravan of thousands of migrants would make its way to the United States.

The active-duty troops added 70 miles of concertina wire to make it more difficult to cross into the country illegally, allowing Customs and Border Protection officials to focus on other tasks, said Vice Adm. Michael Gilday, the director of operations for the Pentagon’s Joint Staff.

Smith asked when it would no longer be necessary to active-duty troops at the border, and Gilday said it is not clear. The Pentagon considered sending the National Guard for the mission and determined they did not have the capacity to handle the mission, Rood said.

“There’s a limit to the number of volunteers, which is the way they have sourced them if they can do it,” Rood said of the National Guard. “The feedback from the National Guard Bureau and the adjutant generals is that at the present state, a little over 2,000 is what they can sustain.”

Trump has in recent days threatened to declare a National Emergency to find funding for his proposed border wall, the center of a political dispute that led to a 35-day shutdown of parts of the federal government that ended Friday. Smith acknowledged Trump has broad authority to declare an emergency, but added that it will be contested in court.

“There are other pots of money, but none of them are that big,” Smith said of how Trump could fund a border wall using Defense Department funding. “Let’s face it: When you look at the discretionary budget, the Department of Defense is where the money is at. So, we’re deeply concerned that if an emergency is declared, that money is going to be taken out of DOD for what some of us think is a questionable purpose.”

Rood declined repeatedly to define which military projects could be scrapped if the president chooses to use Defense Department money to build a border wall.

“The president has not chosen to do so,” Rood said. “Therefore, we have only done the preliminary prudent pre-planning. We have not developed a specific list of military construction projects because the president hasn’t taken that step.”

Paul Sonne contributed to this report.