Donald Trump said he will deploy the US military to the border with Mexico until his "beautiful wall" is finally built.
The US president has become increasingly frustrated by the lack of progress on his signature campaign promise.
In recent days he raged at Mexico for allowing a "caravan" of 1,500 migrants from Central America to walk toward the US border.
Mr Trump said: "We’re going to do some 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 are preparing for the military to secure our border. I think it’s something we have to do."
He added: "We cannot have people flowing into our country illegally, disappearing, and by the way, never showing up for court. Our laws are so weak and so pathetic. It’s like we have no border. We need to have a wall that’s about 700 to 800 miles."
Mr Trump made the announcement during a meeting with Baltic leaders at the White House.
The president said he would later be having a meeting with James Mattis, his defence secretary to discuss the details.
In 2010 President Barack Obama also deployed the military – around 1,200 National Guard troops – to the Mexico border in a temporary move to address drug trafficking. President George W. Bush also did so in 2006.
It marked a further ratcheting up of Mr Trump’s recent rhetoric on immigration.
In recent days the president commented angrily on Twitter about the "caravan" and threatened to upend the North American Free Trade Agreement (Nafta) in retaliation against Mexico.
Amid reports Mexico had on Tuesday started making moves to stop the caravan Mr Trump said: "I’ve just heard the caravan has broken up and Mexico did that, and that’s because I said about Nafta. So, it looks like it’ll be broken up. It makes me very sad it can happen to the US that people can walk into our country."
He added: "The caravan coming in from Honduras thought they were going to walk right through Mexico and through the border. "I said to Mexico ‘I hope you’re going to tell that caravan not to get up to the border’. I think they’re going to do that because, as of 12 minutes ago, it was being broken up."
Mr Trump was angered last month when a giant $1.3 trillion spending bill presented to him by Congress last month contained only $1.6 billion in funding for his border wall, much of it just to repair existing fence segments.
Current US-Mexico border
He also recently declared "dead" the chances of a deal to protect "Dreamer" immigrants who came to the US illegally as children, and accused Democrats of enabling "open borders, drugs and crime".
Mr Trump’s focus on the caravan came after it was highlighted in a report on Fox News.
The caravan is an annual event in which demonstrators protest against violence in Central America, with some in it then attempting to get into the US.
Speaking at a meeting with the leaders of Estonia, Latvia and Lithuania, Mr Trump also said his efforts to make members of Nato contribute more to defence had led to billions of dollars more coming in.
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He said: "A lot of years they (some countries) weren’t paying, they were delinquent. Since I came in many, many billions have come in. They will have to pay more, frankly, they will have to pay more."