His announcement marked Trump's latest attempt to keep the issue of immigration front-and-center in the final stretch before next Tuesday's elections.
Trump and some other Republicans say the right - granted by the Constitution's post-Civil War 14th Amendment - creates an incentive for people to enter the country illegally to have children.
A federal official who was not authorised to be quoted by name said 153 migrants in the second caravan were detained Wednesday during highway inspections in the southern state of Chiapas, a short distance from the Guatemalan border.
Mr Trump reiterated that he wanted to set up "tent cities" to hold people coming to the USA, including those seeking asylum.
For the vast majority of our country's history the military didn't do its work overseas - its number one job was to defend the United States and its borders. Trump has proposed detaining migrants in massive tent cities at the border.
If he follows through on that promise, iIt would be the largest domestic deployment of active duty military in modern American history, and eclipse the number of troops now stationed in Iraq.
In response to a question from a reporter about the military firing on rock-throwing migrants, Trump said this: "No, they don't have to fire".
The first 100 or so active duty troops arrived at the border on Thursday, making initial assessment at the McAllen, Texas, crossing. It is made up of people who have left poverty and violence at home and are heading slowly through Mexico toward the US border.
Most of those being deployed in Operation Faithful Patriot, as the Pentagon has dubbed this massing of forces at the border, are to perform logistical or medical support for U.S. Customs and Border Protection personnel in the border region.
Most of the migrants in the caravans are from Honduras.
President Donald Trump on Thursday attempted to establish immigration as the defining issue of the midterm elections, calling Central American migrants heading to the southern border unsafe and announcing plans to prevent certain asylum seekers from entering the United States.
The Trump administration is being sued for turning away asylum seekers from ports of entry.
It was unclear whether the restrictions Trump was expected to propose would apply only to those traveling in the caravans or extend to all people trying to enter the country.
The Trump administration has been looking at ways to limit the number of asylum seekers.
Senior Trump administration officials have previously encouraged asylum-seekers to make their claims at official ports of entry.
In June, U.S. Attorney General Jeff Sessions said that domestic and gang violence would generally no longer be accepted as reasons for migrants to be granted asylum. Backlogs have grown especially bad in recent months at crossings in California, Arizona and Texas, with people generally waiting five weeks to claim asylum at San Diego's main crossing and sleeping out in the open for days at a time.
USA immigration laws make clear that migrants seeking asylum may do so either at or between border crossings. It's turned out, in fact it was just announced by Homeland Security, you have in just certain areas over 300 people that they know are trouble.
In the past, the practice of metering has resulted in individuals deciding not to endure a lengthy wait to try to get into the country legally and instead to cross illegally.
We don't have to meet the invading army of illegal aliens with tanks to assure that they do not pose a threat to our national security or overwhelm our border patrol and immigration system.