Transcript for Trump proposes potentially using the military to secure US-Mexico border

Rob, thank you. We do move on to president trump's new proposal on border security, vowing to send the military there until he can build the wall. Twice today, the president embellishing that claim, saying he is talking to defense secretary Jim Mattis about it. But the president may have some hurdles to overcome, because using American troops on U.S. Soil requires act of congress. Here's ABC's chief white house correspondent Jonathan Karl. Reporter: The president seemed to catch his own aides by surprise, proposing, potentially, his most aggressive idea yet to secure the border with Mexico -- using the U.S. Military. We have very bad laws for our border. And we are going to be doing some things, I've been speaking with general Mattis, we're going to be doing things militarily,@ until we can have a wall and proper security. We're going to be guarding our border with the military. That's a big step. We really haven't done that before, certainly not very much before. Reporter: A short while later, he was asked to clarify his remarks and said this -- We are preparing for the military to secure our border between Mexico and the united States. We have a meeting on it in a little while with general Mattis and everybody. And I think that it's something we have to do. Reporter: Among the people most surprised by the president's idea to use the military -- the military. Multiple officials tell ABC news there have been no specific proposals yet developed to use the military on the border. But the president has been told recently that he has the authority to deploy national Guard troops. That authority, however, is limited. The military cannot be used for domestic law enforcement such as detaining illegal border crossers. This latest idea comes a week after the president proposed, via Twitter, using the military budget to build the wall. But that idea, almost certainly not within his authority, has been put on the back burner, according to senior white house official. All right, let's get to Jonathan Karl, joining us from the white house. Mexico is responding tonight to the president's proposal? Reporter: The Mexican ambassador to the United States has issued a statement, saying that he is seeking clarification from the state department and the department of homeland security. And Tom, the ambassador adds this. "We share the idea of a secure border, but we do not always agree on how to achieve that goal." Tom? Jonathan Karl for us tonight. Thank you.

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