President Donald Trump made a Thanksgiving Day threat to close the US border with Mexico for an undisclosed period of time.
He said he had given the "OK" for active-duty troops to use lethal force against migrants “if they have to”.
"We're dealing with a minimum of 500 serious criminals," he told reporters during a question-and-answer session with reporters at his Florida golf club, which he dubbed the "southern White House".
He continued: "I'm not going to let the military be taken advantage of. I have no choice. Do I want to happen? Absolutely not, but you're dealing with rough people."
Mr Trump’s border threat came days after a federal judge put the administration’s asylum policy on hold.
Under that new policy, Mr Trump declared no one could apply for asylum except at an official border entry point.
Some ports of entry are already facing huge backups, with people waiting for weeks.
The US government shut down one port of entry, San Ysidro, in California, for several hours early on Monday morning to bolster security amid concerns about a potential influx of migrant caravan members.
Most of the lanes were reopened before the morning rush.
Mr Trump repeated Nielsen’s claim, made earlier this week when she visited a San Diego Pacific Coast beach to see newly installed razor wire wrapped around a towering border wall that cuts across the sand, that there were as many as 500 criminals and gang members in the group heading northwards.
Ms Nielsen refused to answer questions about how they were identified or what crimes they had committed.
Mr Trump asserted that there are “fistfights all over the streets” in Tijuana, Mexico, and that “these are not like normal, innocent people”.
“These are people you talk to them and they start a fistfight,” he said.
“I don’t want that in this country.”
The people of Tijuana “opened up with wide arms” to welcome the caravan, Mr Trump said, and “now they’re going crazy to get them out … because bad things are happening”.
He said if US officials “find that it’s incontrollable, if we find that it gets to a level where we are going to lose control or where people are going to start getting hurt, we will close entry into the country for a period of time until we can get it under control. The whole border.”
In that case, Mexico would take an economic hit, he said, citing an inability to ship cars into the US for sale.
“We’re either going to have a border or we’re not,” Mr Trump said