Defense Secretary Jim Mattis was expected to sign orders either Thursday or Friday for the new deployment, one official said.
Calls for up to 1,000 American troops to be sent to the US-Mexican border have been made by the Department of Homeland Security.
"The Democrat inspired laws make it tough for us to stop people at the border", he added.
This process, known as "return to territory", which was included in Trump's January 2017 executive order on border security, isn't now carried out by Immigration and Customs Enforcement, because Mexico would need to agree to take foreign nationals back. "But they could very well be", Trump said.
Instead, the National Guard troops have conducted some surveillance operations, but they largely have worked in administrative or vehicle maintenance roles behind the scenes, freeing Border Patrol agents to conduct law enforcement operations, according to the Pentagon.
Pentagon spokesman Navy Captain Bill Speaks said the Defense Department was now working with the Department of Homeland Security "to determine the specifics of our support" to border authorities.
Just one day ago, U.S. President Donald Trump tweeted that "criminals and unknown Middle Easterners" were mixed in with a migrant caravan of over 7,000 people that had stopped in southern Mexico.
The issue has become a rallying cry for the U.S. president, who has taken a hard line on illegal immigration and has repeatedly kept the story in the headlines in the run up to America's midterm congressional elections that could see the Democrats regain some degree of power.
Andrew Gillum can dunk, folks
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The network on Friday again aired a repeat of " Megyn Kelly Today " just as it did Thursday following this week's controversy. A Thursday report from the Daily Mail noted that Kelly reportedly will "not ever" come back to NBC after the fallout.
President Donald Trump's administration has expressed alarm at the change in the makeup of migrants attempting to cross into the United States from mostly single adults to children and families traveling together.
However, activists accused Mexico of violating the migrants' rights by detaining anyone who tried to apply for asylum. Caravan leaders said they had expelled some troublemakers from the procession, exhibiting some self-policing.
An illegal alien can claim asylum whether they crossed at a legal port of entry or illegally at any other point along the border. The DHS data released Tuesday goes back to 2013.
Earlier on the day, Trump said Washington will begin cutting off or reducing aid to Guatemala, Honduras and El Salvador, citing migrant caravans heading to the USA border.
Guatemalan Foreign Minister Sandra Jovel dismissed the threat as a posture adopted for the media, telling reporters that the country had received no official notice of the move. Under a full moon early on Thursday, they walked from Mapastepec, close to the Guatemala border in southern Mexico.
They were headed to the town of Pijijiapan, some on trucks but most making the seven hour trek on foot.
As many as 5,000 migrants were reported to have massed in the Mexican city of Tapachula by Sunday evening, some 3,800 kilometers (2,400 miles) from the United States border.
Central American migrants traveling with a caravan to the US make their way to Pijijiapan, Mexico, Thursday, Oct. 25, 2018.