The president said migrants who show up at the official land ports of entry will still be allowed to make claims, but blocking claims by those who sneak into the USA could change the patterns and cut down on the caravans now headed for the border.
Earlier this week President Trump ordered 5800 USA troops to the border to assist Customs and Border protection.
In a lengthy speech on migration, Trump promised an executive order sometime next week that would ban migrants from claiming asylum if they cross the border illegally.
Trump told journalists at the White House that a group of several thousand migrants attempting to walk from Central America through Mexico had thrown rocks "viciously and violently" at Mexican police.
"Migrants seeking asylum will have to present themselves lawfully at a port of entry", Trump said.
"My administration is finalizing a plan to end the rampant abuse of our asylum system", Trump said in a speech about immigration.
USA immigration laws make clear that migrants seeking asylum may do so either at or between border crossings.
In 2014, when there was an increase in families crossing the country's border, the Obama administration detained families who were waiting on asylum claims.
But in recent days, officials from Mexico's immigrant protection agency have organized rides for straggling women and children as a humanitarian effort.
Numbers are changing daily, but so far the Pentagon has authorised 5,239 active-duty troops to go to the border on a 45-day deployment.
His comments came just days before the midterm elections, and seemed created to excite his base as a group of Central American migrants makes its way through Mexico toward the southern US border. An official who spoke on the condition of anonymity said there are about 2,600 USA troops now at staging bases, largely in Texas, with several thousand more expected to flow in through the weekend and move into California and Arizona.
Trump says he hopes USA troops won't fire on the migrants, but says stones and rocks will be considered firearms if thrown.
The number of such immigrants had reached a height of 12.2 million in 2007, representing about 4 percent of the USA population, before declining due in part to a weakening US economy.
-Mexican border could reach 15,000 - roughly double the number the Pentagon said it now plans for a mission whose dimensions are shifting daily.
The Department of Homeland Security's Office of Inspector General said in an October report that they "saw evidence that limiting the volume of asylum-seekers entering at ports of entry leads some aliens who would otherwise seek legal entry into the United States to cross the border illegally".
That's according to three people familiar with the plans who spoke to The Associated Press on Thursday on the condition of anonymity because they weren't authorized to speak publicly.
Under the current Immigration and Nationality Act, "Any alien who is physically present in the United States or who arrives in the United States (whether or not at a designated port of arrival and including an alien who is brought to the United States after having been interdicted in global or United States waters), irrespective of such alien's status, may apply for asylum in accordance with this section".
"You come into the country - you're like two months old ... and you're gonna bring 'em all - your aunts and uncles and grandfathers and lots of people", he said.
Trump has drawn a hard line on immigration just ahead of the midterm elections.
- With reporting from Jill Colvin and Colleen Long.
Trump, who famously instructed former press secretary Sean Spicer to tell reporters his inaugural crowd bested Barack Obama's, made the claim after days of media reports on the substantial masses of people making their way toward the border.
It was reminiscent of the infamous "Willie Horton" ad used against Democratic presidential candidate Michael Dukakis in 1988 and condemned as racist.