President Donald Trump ordered the military deployment in the build-up to last week's midterm elections, with critics assailing the move as a costly political stunt to mobilize his conservative base.

Speaking to reporters at the Pentagon, Mattis said he would visit the border on Wednesday.

The caravan left San Pedro Sula, Honduras, on October 13 and has covered more than 1,500 kilometers (930 miles) so far.

At least two other caravans have since been established, defying threats from Trump -- who has decried what he describes as an "invasion."

The migrants still have hundreds of miles to walk to the border and might not even get there before the scheduled December 15 end date for the military deployment.

The Pentagon said Mattis was traveling to the border for one day and would visit McAllen in southern Texas.

A big question the Defense Department has yet to address is how much the deployment will cost.

The mission could see upwards of about 9,000 active-duty and reserve troops working on or near the border.

It is by far the largest deployment Mattis has overseen since becoming Pentagon chief nearly two years ago.

He said the Pentagon would provide cost updates "as they become known."

Mattis has previously defended the decision to send thousands of soldiers to the border, where they will mainly provide logistical support, rejecting criticism that the move was political.

"We don't do stunts in this department," he said last month.