President Donald Trump has threatened to close the US border with Mexico for an undisclosed period of time if his administration determines that its southern ally has lost "control" on its side.
Trump cited the situation involving migrants camped in Tijuana, Mexico, after travelling in a caravan to reach the US.
Trump called it "a really bad situation" there and said "if we find that it's incontrollable", then "we will close entry into the country for period of time until we can get it under control. The whole border."
The president also said he has given American troops at the border the "OK" to use lethal force against migrants "if they have to".
Trump told reporters: "I hope they don't have to", but he said: "I have no choice" because "you're dealing with rough people".
He also warned there could be a government shutdown next month over security on the border with Mexico, suggesting he could hold up negotiations if no more money is provided for a wall between the two countries.
"Could there be a shut down? There certainly could and it will be about border security, of which the wall is a part," Trump told reporters in Palm Beach, Florida, on Thursday.
Members of a caravan of Central American asylum seekers scale a wall at the US-Mexico border. The president has doubled down on his vow to fence off the border Source: Getty Images
US Defense Secretary Jim Mattis said on Wednesday that he had been granted authority to allow troops on the Mexican border greater powers to help protect border officials, and would await direction from the Department of Homeland Security.
Mattis said troops could help protect the border agents with shields and batons, but would be unarmed.
Earlier, he delivered a Thanksgiving Day holiday message by teleconference to troops in Afghanistan.
Trump told members of the Army, Navy, Marine Corps, Air Force and US Coast Guard in the teleconference that he and first lady Melania Trump wanted to express their "profound gratitude".
He also told American service members on duty around the world, "Your courage truly inspires us."
Trump is spending his holiday in Palm Beach, Florida, at his private Mar-a-Lago club again this year.
He started Thursday morning tweeting as part of his extraordinary public dispute with Chief Justice John Roberts. Trump is warning of "bedlam, chaos, injury and death" if the courts block his efforts to overhaul the nation's immigration laws.
