The Pentagon's decision not to reveal the size of the increase during a hearing before the House Armed Services Committee on Tuesday was blocked by the commission's chairman, Democratic representative Adam Smith of Washington.
"The members of the Committee would have been very interested in discussing what the 3,500 troops going to the border will do in response to the new DHS request," Smith wrote in a letter to Secretary of Defense of the # 39; action, Pat Shanahan.
A defense officer confirmed that the number of additional troops will be approximately 3,500.
"This is a violation of the obligation of the executive section to be transparent with the Congress, which oversees, authorizes and finances its operations," added Smith.
The additional forces will allow the Department of Defense to comply with a request from the Department of Homeland Security that Shanahan approved earlier this month.
"They have sent more troops to the southern border to stop the invasion of the illegal, through the great caravans, in our country. We have stopped the previous caravans, and we will stop them" , Trump wrote.
It is expected that the combined labor force combined with the border will be slightly smaller than that of the 5,900 troops mission.
The Pentagon had previously said that the new troops would be involved in "mobile surveillance and detection, as well as the contract for cable positioning between inlet ports".
The National Security Department is "tracking" three caravans en route to the United States, "one of which has more than 12,000 people in the last estimate," said Secretary of Defense Policy John Rood at the Armed Services Commission of the House.
Defense officials told CNN that the new mobile surveillance would include troops that provided mobile observation messages and vehicles that would involve troops that made a personal radiophanage of customs and border protection to intercept any illegal activity detected .
United States Customs and Border Protection "has requested that additional 150km of concert cable will be awarded no later than March 31," said Rood and the operations director of Joint Service Vice President Michael Gilday , in a joint statement to the Armed Services of the House Committee on Tuesday.
By the end of November, the military had already placed more than 10 miles of barrier obstacles in Texas, Arizona, and California.
This effort is separated from the effort led by the White House 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 historically has involved US military aircraft that CBP personnel want in locations along the border.
The approximately 2,300 active troops deployed at the border were scheduled to return home on December 15, but their implementation extended at the end of January at the request of DHS. Officials say many, but probably not all of these troops will remain until September.
The cost of this implementation was estimated at about 132 million dollars, but this estimate was based on an original end date of January 31.
Additionally, Trump had previously ordered the deployment of the National Guard forces to help secure the border. There are about 2,200 national guardians assigned to this mission. This deployment, which is also scheduled to end in September, estimates that it will cost $ 550 million.
The Pentagon has refused to tell where the money comes from to pay the deployments.
The implementation of active troops for border security has been questioned by lawmakers.
"The deployments on the border seem to conflict with the declared efforts of the Department (Defense) to rebuild the preparation," said Adam Smith, chairman of the Chamber's Armed Services Committee on Tuesday.
"This deployment on the southern border seems to aggravate this problem by further interrupting the training cycles of the unit," he added.