US authorities on Monday said more than 5,200 troops would head south to bolster border security, in a bid to prevent a caravan of Central American migrants from illegally crossing.

General Terrence O'Shaughnessy, head of the US military's Northern Command, told Pentagon reporters Tuesday that the precise number is currently 5,239 troops -- and that the figure would grow.

"What I can confirm is there will be additional force over and above the 5,239. The magnitude of that difference, I don't have an answer for it now," he said.

President Donald Trump in recent weeks has repeatedly said more troops are needed to tighten border security, and he has made political capital of the caravan ahead of crucial midterm congressional elections that could see the Democrats regain some degree of power.

In April, Trump ordered up to 4,000 National Guardsmen to head to the border as a different migrant caravan wound its way north. About 2,100 have deployed.