Members of a migrant caravan walk into the interior of Mexico after crossing the Guatemalan border on October 21, 2018 near Ciudad Hidalgo, Mexico (Photo by John Moore/Getty Images)

(CNSNews.com) - As thousands of Central Americans continue marching north, President Trump reportedly plans to make a major immigration speech on Tuesday.

According to the Washington Post, he is expected to announce that he is invoking emergency powers, on national security grounds, to stop those people and others from entering the United States.

Trump on Monday had a message for the thousands of Central Americans heading to the Southwest border:

"Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border," he tweeted. "lease go back, you will not be admitted into the United States unless you go through the legal process. This is an invasion of our Country and our Military is waiting for you!"

"I think what the president has been saying and will continue to say, and certainly what I have been saying, is, this caravan is not getting in," Homeland Security Secretary Kirstjen Nielsen told "Fox News Sunday."

"There is a legal way to enter this country. Those who choose to enter illegally will be stopped."

Nielsen said her message to the caravan is, "Do not come. You will not be allowed in. There is a right way to immigrate to the United States and this is not it."

What we're really talking about is the flow of people that are headed towards United States. They have chosen to break laws along the way. You saw some of them, frankly overwhelm and burst through the border between Guatemala and the country of Mexico. Mexico has offered them asylum. In some cases, they have refused. Mexico has offered them work permits. In some cases, they have refused. And I think what the president and I are both saying, and we want to be clear on this is, if you seek asylum, do so in the first place, country, Mexico has offered you refuge. If you want a job, that is not asylum. If you want to be reunited with your family, that is not asylum. If you want to just come live in the United States, that is not asylum; there are legal ways to do that. But this is about the rule of law. This is about understanding who is in the flow. And Chris, I cannot tell you as Secretary of Homeland Security, that I know every person in this flow.

Nielsen described the daily Southwest border breaches as a "crisis":

"We are stopping between 1,500 and 1,700 people a day, trying to cross illegally into this country. This caravan is one iteration of that but frankly we essentially see caravans every day with these numbers.

"So I think what the president is making clear is every possible action, authority, executive program, is on the table to consider, to ensure that it is clear that there is a right and a legal way to come to this country and no other ways will be tolerated."

Host Chris Wallace asked Nielsen how women and children "threaten national security."

"Well I think there's a couple ways to look at this," Nielsen responded. She said the caravan "isn't a ticketed event," and it may include terrorists who blend in with the crowd.

"In general, we stopped -- across United States -- 10 known or suspected terrorists a day from getting into the United States."

Last week, Trump tweeted: "To those in the Caravan, turnaround, we are not letting people into the United States illegally. Go back to your Country and if you want, apply for citizenship like millions of others are doing!"

The American Civil Liberties Union, meanwhile, issued a statement last week, commenting on Trump's reported plan to block asylum-seekers and immigrants from crossing the U.S.-Mexico border:

“It’s disgraceful the Trump administration would even consider what’s being reported. It would mean refusing to protect people who can prove they are fleeing persecution. That would be a huge moral failure and any plan along these lines will be subject to intense legal scrutiny.”