President Donald Trump has vowed to send the US military to secure the nation’s southern border with Mexico.

“We are going to be doing things militarily,” Mr Trump said at the White House on Tuesday, adding that it would be a “big step”.

Earlier Mr Trump threatened to cut aid to Honduras as a “caravan” of asylum seekers makes its way north to the US.

Both of his predecessors in the White House also deployed National Guard troops to help secure the US border.

Image copyright Getty Images Image caption The border, including the Rio Grande River, is policed by US Border Patrol

President Barack Obama omce sent 1,200 troops to guard the boundary, while President George W Bush deployed about 6,000 troops to help Border Patrol in what was called Operation Jump Start.

During a working lunch with leaders of Baltic countries on Tuesday, Mr Trump told reporters the North American Free Trade Agreement (Nafta) was at risk unless Mexico stopped the flow of migrants.

He tweeted earlier for the third day about the “caravan” of migrants heading north from Central America.

“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 the border.”

“Cash cow Nafta is in play, as is foreign aid to Honduras and the countries that allow this to happen.

Mr Trump began tweeting on Sunday about the “caravan” of about 1,100 mostly Honduran immigrants, following a report on Fox News’ morning show that used the term.

The group, which has been marching along roadsides and railways in the southern Mexican state of Oaxaca, is being organised by US-based Pueblo Sin Fronteras (People Without Borders).

The migrants gathered in the south of the country on 25 March. The Mexican government said on Monday evening around 400 of the migrants had already been sent back home.

Mexican officials told Reuters news agency they were determining whether the remaining migrants had the legal right to remain in Mexico.

According to recent reports, the caravan stopped at a football field as participants decided whether to continue 800 miles north to the US, return south, or seek asylum in Mexico.

Image copyright Getty Images Image caption Migrants taking part in the caravan to the US queue for food in Oaxaca, Mexico on Tuesday Image copyright Getty Images

Constructing a “big, beautiful wall” along the Mexican border was a signature campaign promise for Mr Trump, but so far the plan to erect a new physical barrier has been thwarted by lawmakers and appears to have stalled.

A major government spending bill which he signed last month included $1.6bn for the border wall – far short of the $25bn the White House sought.

And there were strings attached to the funding Congress did approve. Most of it can only be used to repair stretches of the border where there already is a wall, not to build new segments.

Last month the Pentagon confirmed Mr Trump had held “initial” talks with Defence Secretary James Mattis about using some of the Pentagon’s budget for constructing a wall.

Two Democratic senators wrote to the defence secretary on Monday saying his department had “no legal authority” to use its funds for such a purpose.

“Such a controversial move could only be funded by cutting other vital priorities for our service members,” wrote Senators Dick Durbin and Jack Reed.