Updated

President Trump on Tuesday said that the U.S. will secure the southern border with the military until a wall can be built, calling the move a “big step.”

Trump made the remarks during a meeting with Baltic leaders, where he said he had discussed the matter with Defense Secretary Jim Mattis. A White House official revealed later Tuesday to Fox News that the plan considered by Trump would be a “substantial” mobilization of the National Guard.

“Until we can have a wall and proper security, we’re going to be guarding our border with the military,” he said. “That's a big step, we really haven’t done that before, or certainly not very much before.”

At a news conference later, he confirmed the plan, saying the border is unprotected by “our horrible, horrible and very unsafe laws.”

“We don't have laws, we have catch-and-release,” he said. “You catch and then you immediately release and people come back years later for a court case, except they virtually never come back.”

Trump did not offer specifics, but the move appears to be at least partly motivated by a caravan of over 1,000 Central American migrants heading toward the U.S. border. Buzzfeed, which first reported on the caravan, said that Mexican officials had not yet attempted to stop the flow.

In Tuesday's discussion, Trump and senior officials “also agreed on the need to pressure Congress to urgently pass legislation to close legal loopholes exploited by criminal trafficking, narco-terrorist and smuggling organizations,” White House Press Secretary Sarah Sanders said. She added that Mattis, Homeland Security Secretary Kirsten Nielsen, Attorney General Jeff Sessions, Joint Chiefs Chairman Gen. Joseph Dunford and Chief of Staff John Kelly were among the other officials present.

Reports of the caravan angered Trump, who has sent out a number of tweets threatening to end the North American Free Trade Agreement (NAFTA) and to cut foreign aid to countries such as Honduras, from where many of the migrants originate.

“The big Caravan of People from Honduras, now coming across Mexico and heading to our “Weak Laws” Border, had better be stopped before it gets there,” he tweeted Tuesday. “Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW!”

At the press conference he said that NAFTA was an “embarrasing” deal and that he had told Mexican officials on Monday: “I hope you're going to tell that caravan not to get up to the border.” He added that he thinks the caravan was being broken up as a result.

The Pentagon was scrambling to come up with a response to Trump's statement on the military guarding the border. But according to a memo obtained by Fox News and discussions with officials, one area where the Pentagon could contribute immediately is the Air Force’s Barry Goldwater live-fire range, which shares a 35-mile border with Mexico in southern Arizona.

The Defense Department is already offering some support to the border, including U.S. Navy ships patrolling waters to seize drugs as well as missions involving over 100 personnel from U.S. military’s northern command -- including eight planes and a drone -- to help border patrol.

In 2010, President Obama authorized 1,200 National Guard troops to increase security at the border, although Republican lawmakers questioned their role and whether they would in fact be “boots on the ground."

Trump has struggled to secure funding for his central campaign promise of a wall, which the administration and border officials say will cost approximately $20-25 billion. In the omnibus bill signed by Trump last month, $1.6 billion was included for technology and some replacement of existing border fencing -- although it excluded the prototypes Trump recently viewed in California.

Trump briefly considered vetoing the legislation both over its lack of funding for the wall and also the failure to include a fix for the expiring Deferred Action for Childhood Arrivals (DACA) program that granted protection for illegal immigrants brought to the country as children. He has since floated the idea of getting the Pentagon to fund the wall.

Brandon Judd, President of the National Border Patrol Council, said on Fox News Radio’s “The Todd Starnes Show” that it gives Border Patrol “certainty of apprehension.”

“The criminal smugglers, this is a multibillion dollar industry. They smuggle humans, they smuggle drugs,” he said. “This criminal enterprise, if we arrest the majority of people that cross the border illegally, we put a dent into their criminal enterprise, and if you put a dent into their criminal enterprise, then you can possibly stop them.”

Fox News' Kristin Brown, John Roberts, Lucas Tomlinson and Wes Barrett contributed to this report.