(CNN) President Donald Trump said Thursday he has authorized American troops on the US-Mexico border to "use lethal force" if necessary against an approaching group of migrants while also threatening to close "the whole border."

Trump, who was speaking with military members and reporters at Mar-a-Lago, also said there "certainly could" be a government shutdown over border wall funding in December.

"If they have to, they're going to use lethal force. I've given the OK," Trump said. "If they have to -- I hope they don't have to."

"I have no choice," Trump said, and, without providing evidence, added, "You're dealing with a minimum of 500 serious criminals" and "rough people."

Earlier this week, Trump approved a memorandum that granted new authority to troops on the border to protect Customs and Border Protection personnel from migrants if they engage in violence. Until the new authority was granted, troops were not allowed to intervene if CBP personnel came under attack unless they needed to act in their own self-defense.

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