"By the end of the week, we will deploy over 5,200 soldiers to the southwest border," Air Force General Terrence O'Shaughnessy, head of the US military's Northern Command, told reporters.

Trump in recent weeks has repeatedly said more troops are needed to tighten security at the border, and he has made political capital of the caravan ahead of important mid-term congressional elections that could see the Democrats regain a degree of power.

The United Nations estimates about 7,000 people have joined the caravan since it set out from San Pedro Sula, Honduras, on October 13.

"They are incentivized to try to cross our border by the gaps in our legal framework and the expectation that they would be allowed to stay," said US Customs and Border Protection Commissioner Kevin McAleenan.

O'Shaughnessy said the military deployment would focus first on trying to "harden" border crossings and surrounding areas, with the work done by combat engineering battalions with experience building temporary fencing.

Additionally, the Pentagon is sending three helicopter companies with aircraft equipped with high-tech sensors and night-vision capabilities.

They will bring border "personnel exactly where they need to be, regardless of the conditions," he said.

Military police units are also being deployed.

Though soldiers are not conducting direct law-enforcement operations and will ostensibly be in a support role, they will nonetheless be deployed with their weapons, officials noted.