(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.

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