Donald Trump said on Thursday he has authorised US troops to "use lethal force" against migrants attempting to cross America’s southern border if necessary.
The US president also threatened to close the entire border with Mexico if officials were unable to maintain control over ports of entry into the US.
"If they have to, they have to use lethal force. I hope they don’t have to, but you’re dealing with a minimum of 500 serious criminals. So I’m not going to let the military be taken advantage of," he told reporters.
Mr Trump continued: "If we find that it gets to a level where we lose control or people are going to start getting hurt, we’re going to close entry into the country for a period of time until we get it under control."
He added that “large numbers of people are forming at our border…in many cases, we have no idea who they are, and in many cases, they’re not good people. They’re bad people.”
The comments followed reports that John Kelly, Mr Trump’s chief of staff, signed an order on Tuesday enabling active troops to use “a show or use of force (including lethal force, where necessary), crowd control, temporary detention and cursory search” where necessary to protect border agents.
Leaked documents seen by the Military Times website state the order allows “Department of Defense military personnel” to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary” to protect border agents.
The move is likely to provoke concern from military and legal officials, who have previously warned such action could be unlawful.
Mr Trump caused alarm earlier this month when he suggested troops deployed to the Mexican border could open fire at migrants approaching America if they throw rocks.
A number of retired generals came forward to publicly criticise the comments, suggesting that lethal force could violate the rules of engagement.
Mr Trump deployed 5,800 troops on the US-Mexico border shortly before the midterm elections, bringing the first wave back again this week – the deployment having cost $72 million.
The president also used a traditional Thanksgiving message to American troops on Thursday to threaten to close the US-Mexico border.
Mr Trump, who is spending the holiday at his Florida estate, Mar-a-Lago, spoke by telephone with the commander of Bagram air force base in Afghanistan, telling the troops he was “profoundly grateful” for their service.
He then moved on to continue his attacks on the Ninth Circuit Court. Judge Jon Tigar on Monday enraged the president by blocking his plan to prevent migrants who crossed illegally from seeking asylum.
"It’s a terrible thing when judges take over your protective services when they tell you how to protect your border,” he said. “It’s a disgrace.”
Mr Trump also defended his daughter Ivanka, after it emerged earlier in the week she has used a private email address for government business – something he repeatedly attacked Hillary Clinton for – and repeated his defence of Saudi Arabia, in the light of their murder of journalist Jamal Khashoggi.
"I don’t know if anyone’s going to be able to conclude that the Crown Prince did it," said Mr Trump – despite the CIA connecting Mohammed bin Salman to the killing.
"Whether he did or whether he didn’t, he denies it vehemently."
On Thursday the president, who is now facing increasing questions about his failure to visit troops in any warzone, hinted during the Thanksgiving call that he may soon travel to Afghanistan. "Maybe I’ll see you over there. You never know what’s going to happen," he told the commander.
Mr Trump, after the call, paid a 12 minute visit to the Coast Guard, wishing the assembled crowd a happy Thanksgiving and praising them for their work in hurricane responses, saying they had saved thousands of lives since he became president.
He spent the rest of the holiday playing golf at his Florida course, arriving shortly before midday.