Reporting by CBS News' David Martin and Kathryn Watson

Defense Secretary Jim Mattis is preparing an order order that would send approximately 800 U.S. troops to the border, CBS News' David Martin has confirmed, as President Trump's frustration builds over the caravan of migrants approaching the U.S.

That is in addition to the National Guard troops sent earlier this year, although it is not yet clear whether the additional troops will also be National Guard or active duty military members, Martin reports. In April, the Pentagon authorized 4,000 National Guard troops to head to the southern border to aid U.S. and Customs and Border Protection. CNN first reported the Pentagon's intent to sign off on more troops.

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The additional troops will not have any authority to apprehend illegal immigrants but will be able to erect barriers and provide reconnaissance searching for illegal border crossers. It's not yet exactly clear where they will deploy.

The move comes as Mr. Trump claimed on Twitter Thursday morning he is "bringing out the military for this National Emergency." The president, who has long blasted illegal immigration and touted the importance of border security, has focused on the caravan in recent days leading up to the midterms, calling the matter a great Republican issue ahead of the midterms.

"Brandon Judd of the National Border Patrol Council is right when he says on @foxandfriends that the Democrat inspired laws make it tough for us to stop people at the Border. MUST BE CHANDED [SIC], but I am bringing out the military for this National Emergency. They will be stopped!" the president tweeted Thursday morning.

Mr. Trump has fumed over the caravan of thousands approaching the U.S., claiming "unknown Middle Easterners" and criminals are in the crowd. Pressed on what proof he had "Middle Easterners" were in the crowd, the president admitted earlier this week that there "is no proof."

"There's no proof of anything," the president told reporters Tuesday in the Oval Office. "There's no proof of anything. But there could very well be."

The president has also said he will cut off aid to some Central American countries for failing to stop the migrants within their borders.

A White House official would not comment on the report at this time.