The Pentagon is deploying 5,200 troops along the U.S. border with Mexico to help agents deal with a caravan of Honduran migrants heading to the United States.
"Border security is national security," General Terrence O'Shaughnessy, head of U.S. Northern Command, told reporters Monday.
He said the 5,200 soldiers will be in addition to the nearly 2,100 National Guardsmen already along the border.
Customs and Border Protection Commissioner Kevin McAleenan said there are currently about 3,500 men, women and children in Mexico slowly making their way to the U.S.
A second caravan of about 3,000 approached the Guatemalan-Mexican border Sunday with the apparent intention to meet up with the larger group.
Some of the migrants crossed the river into Mexico despite police efforts to keep them from entering the country as one large group.
McAleenan said those who manage to make it into the U.S. will be apprehended and their asylum requests considered, as required by U.S. law. He said border agents will act with the highest respect for the law and will treat people as humanely as possible.
But McAleenan said there is no benefit in trying to get in as part of a large group, especially in what he calls an "illegal and unsafe" manner.
He said U.S. immigration courts are facing a huge backlog of asylum requests and reminded those in the caravan that they already have generous protections in Mexico, including the promise of work visas and the chance to apply for refugee status.
McAleenan said 1,900 asylum-seekers are picked up at U.S. border crossings a day, many of whom put themselves in the hands of violent human smugglers.
President Donald Trump put it more bluntly when he tweeted Monday, "This is an invasion of our country and our military is waiting for you."
Again, without providing evidence, he said there are "many gang members and some very bad people mixed into the caravan."
The president also claims "Middle Easterners" are in the group.
The first group of migrants left San Pedro Sula, Honduras, earlier this month. Honduras is one of the world's most violent and deadliest nations, in part, because of gangs and drugs.
Many of those trying to get to the United States want to escape the daily threat to their lives and the lack of jobs, and want to get their children away from the influence of gangs.
Trump said anyone wanting to get into the United States must do so through the legal process.
But news reports quote unnamed U.S. officials as saying the White House is weighing a range of administrative and legal actions on grounds of national security to restrict the ability of migrants to seek asylum.
Although no decision has reportedly been made, immigration attorneys told VOA the move would be quickly challenged in court.
The U.N. refugee agency is urging Washington to allow people fleeing persecution and violence to request asylum on U.S. territory.
"Our position globally is that the individuals who are fleeing persecution and violence need to be given access to territory and protection, including refugee status and determination procedure," UNHCR spokesman Andrej Mahecic told VOA.
"And if the people who are fleeing persecution and violence enter Mexico, they need to be provided access to the Mexican asylum system, and those entering the United States need to be provided access to the American asylum system."
Lisa Schlein and Aline Barros contributed to this report.