US President Donald Trump inspects border wall prototypes with Chief Patrol Agent Rodney S. Scott in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN        (Photo credit should read MANDEL NGAN/AFP/Getty Images)
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US President Donald Trump inspects border wall prototypes with Chief Patrol Agent Rodney S. Scott in San Diego, California on March 13, 2018. / AFP PHOTO / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)
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(CNN) —  

Over 3,000 additional active duty troops will be deployed to the southern US border to bolster security, joining the 2,300 troops already there, several defense officials tell CNN.

The Pentagon’s decision to not reveal the size of the increase during a hearing before the House Armed Services Committee on Tuesday was slammed by the committee’s chairman, Democratic Rep. Adam Smith of Washington.

“The Members of the Committee would have been extremely interested in discussing what the 3,500 troops going to the border in response to DHS’s latest request will be doing there,” Smith wrote in a letter to acting Secretary of Defense Pat Shanahan.

A defense official confirmed the number of additional troops will be approximately 3,500.

“This is a violation of the executive branch’s obligation to be transparent with Congress, which oversees, authorizes, and funds its operations,” Smith added.

The additional forces will allow the Department of Defense to fulfill a Department of Homeland Security request for assistance that Shanahan approved earlier this month.

President Donald Trump confirmed that additional troops would be sent via a tweet on Thursday.

“More troops being sent to the Southern Border to stop the attempted Invasion of Illegals, through large Caravans, into our Country. We have stopped the previous Caravans, and we will stop these also,” Trump wrote.

The combined active duty force at the border is expected to be slightly smaller than the mission’s peak of 5,900 troops.

The Pentagon had previously said that the new troops would be involved in “mobile surveillance and detection, as well as concertina wire emplacement between ports of entry.”

The Department of Homeland Security is “tracking” three caravans en route to the United States, “one of which is over 12,000 people in the latest estimate,” Under Secretary of Defense for Policy John Rood told the House Armed Services Committee Tuesday.

Defense officials have told CNN that the new mobile surveillance would include troops manning mobile observation posts and vehicles that would involve the troops radioing Customs and Border Protection personnel to intercept any detected illegal activity.

US Customs and Border Protection “has requested that an additional 150 miles of concertina wire be emplaced no later than March 31,” Rood and the Director of Operations for the Joint Staff Vice Adm. Michael Gilday wrote in a joint statement to the House Armed Services Committee on Tuesday.

By the end of November, the military had already placed over 10 miles of wire obstacles in Texas, Arizona and California.

This effort is separate from the White House-led effort to potentially use existing Pentagon funds and personnel to help build new sections of a border wall.

US troops will also continue to provide aviation support to Customs and Border Protection, which has historically involved US military aircraft flying CBP personnel to locations along the border.

The approximately 2,300 active-duty troops currently deployed to the border were originally scheduled to come home on December 15 but their deployment was extended to the end of January at the request of DHS. Officials say that many but likely not all of those troops will now stay until September.

The cost of that deployment was estimated at about $132 million, however that estimate was based on an original end date of January 31.

Additionally, Trump had previously ordered the deployment of National Guard forces to help secure the border. There are about 2,200 National Guardsmen assigned to that mission. That deployment, which also is scheduled to end in September, is estimated to cost $550 million.

The Pentagon has declined to say where the money to pay for the deployments is coming from.

Deploying active-duty troops for border security has been questioned by lawmakers.

“The deployments to the border seem to conflict with the (Defense) Department’s stated efforts to rebuild readiness,” Rep. Adam Smith, the Chairman of the House Armed Services Committee said Tuesday.

“This deployment to the southern border seems to exacerbate that problem by further disrupting unit training cycles,” he added.