A Pentagon statement said the support would come in the form of logistical and engineering assistance, including the construction of "temporary barriers, barricades and fencing."

The Pentagon did not provide details on how many troops would be sent, but US officials on Thursday told AFP that about 800 service members would be deployed to the frontier.

"After receiving a request for assistance from the Secretary of Homeland Security, the Secretary of Defense has approved providing mission-enhancing capabilities to Department of Homeland Security, US Customs and Border Protection along the Southwest Border," the Pentagon statement read.

The troops would augment the 2,100 or so National Guardsmen who were deployed in April to support border operations, and could come from multiple military bases around the US.

Other help would include aviation support to ferry border agents around and medical to teams to "triage, treat and prepare for commercial transport of patients," the statement read.

The military beef-up of the border comes as thousands of Central American migrants are crossing Mexico toward the United States in a slow-moving caravan.

The issue has become a rallying cry for Trump, who has taken a hard line on illegal immigration and has repeatedly kept the story in the headlines in the run up to America's midterm congressional elections that could see the Democrats regain some degree of power.