WASHINGTON — President Trump said on Tuesday that he planned to order the military to guard parts of the southern border until he can build a wall and tighten immigration restrictions, proposing a remarkable escalation of his efforts to crack down on migrants entering the country illegally.

Mr. Trump, who has been stewing publicly for days about what he characterizes as lax immigration laws and the potential for an influx of Central American migrants to stream into the United States, said he had been discussing with Jim Mattis, the secretary of defense, about resorting to military deployments. “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,” Mr. Trump said at the White House, seated beside the defense secretary at a meeting with visiting leaders of Baltic nations. “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, or certainly not very much before.”

Mr. Trump’s comments came after he kicked off his third consecutive day of tweeting about America’s “weak” border laws on Tuesday and called on Congress to act, following a new push for legislation to enforce immigration laws for those living illegally in the United States.

Mr. Trump’s Twitter thread on immigration policy started Sunday and, since then, he has consistently threatened to pull out of the North American Free Trade Agreement, known as Nafta. On Tuesday, Mr. Trump said Nafta “is in play,” and repeated his contention that Nafta was a “cash cow” for other nations.

The president’s tweets do not always lead to a new policy, but on Monday afternoon the White House announced Mr. Trump’s new push for legislation to make it more difficult to enter and stay in the United States illegally. It was not immediately clear what impact Tuesday’s tweet would have on immigration policy or Nafta negotiations.

The president on Tuesday also raised the issue of the “caravans” of people he says are headed to the United States through Mexico. The caravan has been a popular topic on Fox News — the president’s favorite news network. The president of the National Border Patrol Council, Brandon Judd, said recently on “Fox and Friends” that America’s current immigration policy is luring a caravan of Central American migrants to the United States.

A group called Pueblo Sin Fronteras organized the caravan, which consists of about 1,200 people — including infants, the elderly and people facing violence in their homelands. The group predicted it would be at least a month before the caravan arrived at the United States border with Mexico. Roberto Corona, the group’s communications director, said the group will be much smaller by the time it reaches the American border, because many of the migrants know they would not qualify for asylum in the United States and would instead be turned away. On Tuesday, Mr. Trump warned that “the big Caravan of People from Honduras” must be stopped before it gets to the border.

Late Monday, Mexican immigration officials started to negotiate with the caravan’s organizers. And Mexican authorities have agreed to provide humanitarian visas to the migrants so that they can stay in Mexico legally, a representative from Pueblo Sin Fronteras said.

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