Donald Trump says he is finalising a plan to force migrants to present themselves at a legal port of entry to get into the US.
The president, speaking in the Roosevelt Room of the White House, told reporters: "We have no choice.
"We will defend our borders and we will defend our country."
He added: "No nation can allow itself to be overwhelmed by uncontrolled people rushing our border."
Mr Trump, whose core supporters back a tough line on immigration, also repeated his claim that a caravan of Central American migrants heading to the US-Mexico border is "like an invasion".
The president added that the "illegal aliens" should "turn back now".
Mr Trump has unveiled his immigration policy ahead of the midterm elections on 6 November.
His successful 2016 presidential campaign had a strong focus on border security.
Mr Trump told reporters on Thursday that immigrants "cost our country billions and billions of dollars each year", adding that they put an "enormous strain on our local schools".
The president said earlier on Thursday that the number of troops deployed to the US-Mexico border could reach 15,000.
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The figure took the Pentagon by surprise as it had only planned to send 7,000 troops to support Customs and Border Protection agents.