US President Donald Trump has vowed to send the US military to secure the nation’s southern border with Mexico until a long-promised wall is completed and “proper security” is in place.

Speaking at a White House lunch with leaders of Baltic states, Mr Trump said he has been discussing the idea with his Defence Secretary, Jim Mattis.

Mr Trump, who earlier threatened to halt US foreign aid to Honduras and other countries unless they stopped a “caravan” of Central American migrants heading to the US, reiterated threats to derail the North American Free Trade Agreement (NAFTA) if the migrants are not stopped.

He called the use of the military at the southern border “a big step”.

“We will be doing things with Mexico, and they have to do it, otherwise I’m not going to do with the NAFTA deal,” Mr Trump said.

WE WILL PROTECT OUR SOUTHERN BORDER! pic.twitter.com/Z7fqQKcnez

— Donald J. Trump (@realDonaldTrump) April 3, 2018

He added that the US border laws “are so weak and so pathetic … it’s like we have no border”.

“Until we can have a wall and proper security we’re going to be guarding our border.”

In a post on Twitter earlier on Tuesday, Mr Trump tweeted: “The big Caravan of People from Honduras, now coming across Mexico and heading to our ‘Weak Laws’ Border, had better be stopped before it gets there.”

“Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW!”

The big Caravan of People from Honduras, now coming across Mexico and heading to our “Weak Laws” Border, had better be stopped before it gets there. Cash cow NAFTA is in play, as is foreign aid to Honduras and the countries that allow this to happen. Congress MUST ACT NOW!

— Donald J. Trump (@realDonaldTrump) April 3, 2018

It is unknown how many troops Mr Trump would send.

President Barack Obama sent nearly 1200 soldiers to guard the border, while President George W. Bush deployed about 6000 troops.

Some members of Congress said they were uncomfortable with the idea of using the military at the border. Democratic Senator Brian Schatz said Mr Trump should have to ask for approval from Congress.

“I think we should put that new law to a vote in the Senate as soon as possible,” he said on Twitter. “I predict fewer than 20 votes.”

Republican Representative Francis Rooney, a member of the Foreign Affairs Committee in the US House of Representatives, said there was a risk of increased violence.

-with AAP