US President Donald Trump says he has authorised American troops to use “lethal force” against an approaching group of aspiring migrants, while threatening to close the entire Mexican border indefinitely. 

“If they have to, they’re going to use lethal force. I’ve given the OK,” Mr Trump told reporters on the US Thanksgiving holiday at his Mar-a-Lago resort in Florida.

“If they have to – I hope they don’t have to,” he said.

Mr Trump has sent almost 6000 troops to the Mexico border to guard against a group of migrants approaching the US through Mexico from Central America.

Many of the migrants have said they are seeking asylum and are fleeing persecution, poverty and violence in their home countries.

The growing caravan of asylum seekers includes many hundreds of children,

Almost 3000 migrants have arrived in the city of Tijuana near the US border and the town’s mayor has said he expects the number to reach 10,000 in the coming weeks.

Mr Trump said he has no choice but to take a hard line against the migrants, adding, without providing evidence, that, “you’re dealing with a minimum of 500 serious criminals” and “rough people”.

The President also said he would order the closure of the US-Mexico border for an undisclosed period of time if it is determined that America’s southern neighbour has lost “control” of the situation.

Mr Trump described a growing group of of migrants camped in Tijuana near the US border as “a really bad situation” .

He said that if the US find’s the migrant gathering in Mexico to be “uncontrollable”, then “we will close entry into the country for period of time until we can get it under control. The whole border”.

US Defence Secretary Jim Mattis on Wednesday confirmed 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.

Mr Mattis said troops could help protect the border agents with shields and batons, but would be unarmed.

Earlier, Mr Trump delivered a Thanksgiving holiday message by teleconference to troops in Afghanistan, hinting he may visit them for the first time.

Speaking to the commander of Bagram Air Field in Afghanistan the President expressed his gratitude to the troops and added: “Maybe I’ll see you over there. You never know what’s going to happen.”

Asked by a reporter if he wanted to go to Afghanistan, Mr Trump said:  “We’ll be doing some interesting things.”

Mr Trump is again spending the Thanksgiving holiday at his private Mar-a-Lago club in Palm Beach, Florida.

He started the morning tweeting as part of his extraordinary public dispute with Chief Justice John Roberts.

Mr Trump is warning of “bedlam, chaos, injury and death” if the courts block his efforts to overhaul the nation’s immigration laws.

-with agencies