The US military will deploy more than 5200 troops, including armed soldiers, to bolster President Donald Trump’s efforts to secure the border with Mexico as a caravan of migrants treks toward the frontier.

General Terrence O’Shaughnessy, the head of US Northern Command, said 800 US troops were already on their way to the Texas border and 5200 would be headed to the south-west region by week’s end, far higher than the 800 to 1000 initially forecast.

“That is just the start of this operation. We will continue to adjust the number and inform you of those,” General O’Shaughnessy said on Monday (local time).

“But please know that is in addition to the 2092 that are already employed from our National Guard” troops.”

The Pentagon previously declined comment on potential troop numbers, saying that planning was still under way for a mission that risks drawing the US military into a politically charged operation just days ahead of the November 6 mid-term congressional elections.

Mr Trump, who has seized on the Central American migrant caravan in campaign rallies ahead of the vote, said on Twitter that the military would be waiting for the procession – suggesting a far more direct role in confronting the migrants than US defence officials have previously suggested.

“Many Gang Members and some very bad people are mixed into the Caravan heading to our Southern Border,” Mr Trump tweeted.

“Please go back, you will not be admitted into the United States unless you go through the legal process. This is an invasion of our Country and our Military is waiting for you!” he added.

A caravan of mostly Hondurans, estimated to number between 3500 and 7000, who left their country in mid-October is now in southern Mexico. Since then, two more caravans of migrants have set off.

Some migrants have abandoned the journey, deterred by the hardships or the possibility instead of making a new life in Mexico. Others joined it in southern Mexico.

There are already 2100 US National Guard forces at the border, sent after a previous Trump request in April.

US officials told Reuters last week the military would have not have an active law enforcement role, instead sending engineers, pilots and other support staff, including some that might be able deal with crowd control on the US side of the border.

Last week, US Defence Secretary Jim Mattis authorised the military to provide “mission-enhancing capabilities” to Customs and Border Protection to help build temporary barriers and housing.

-AAP