US President Donald Trump said today that he would deploy as many as 15,000 military personnel to the border with Mexico in response to a caravan of Central American migrants making its way northward.

That doubles the figure that Pentagon officials said would be headed there.

"We'll go up to anywhere between 10 and 15,000 military personnel on top of Border Control, ICE and everybody else at the border," Trump said in remarks to reporters before departing Washington for a campaign rally in Florida.

"Nobody's coming in. We're not allowing people to come in."

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Trump's comments came a day after the general in charge of the border deployment said 5239 active-duty troops would be heading to the border with more potentially to follow, in addition to the 2092 members of the National Guard already there.

If the deployment reaches 15,000 troops, it would be roughly equivalent to the size of the US military's presence in Afghanistan and three times the size of the presence in Iraq.

Already, the deployment is believed to be the largest of its kind in more than a century.

The decision to deploy such a large number of troops in the days before the Midterm elections has resulted in accusations from critics of the President that he is using the issue as a political stunt designed to fire up a base concerned about immigration.

Defence Secretary Jim Mattis, speaking to reporters during a visit by the South Korean defence minister to the Pentagon, rejected the criticism, saying that the military was deploying to support the Department of Homeland Security and Customs and Border Protection.

"The support that we provide to the Secretary for Homeland Security is practical support based on the request from the commissioner of customs and border police, so we don't do stunts in this department," Mattis said.

The Defence Secretary said the military had carried out similar missions within the United States following natural disasters. "We are there in support of the Secretary of Homeland Security, who needs additional military assistance," he said.