"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," Mattis told Pentagon reporters.

"We don't do stunts in this department."

Numbers are changing daily, but so far the Pentagon has authorized more than 5,200 troops to go to the border after President Donald Trump said he wanted to stop an "invasion" of migrants making their through Mexico toward the US.

Another 2,000 troops are on standby, and some 2,000 National Guardsmen are already in the border region to support agents there.

It's a massive deployment, the largest since Mattis came into office nearly two years ago, and critics have said Trump only ordered the move to score political support.

He has repeatedly said more troops are needed to tighten border security, and has made political capital of the migrant caravan in Mexico ahead of crucial midterm congressional elections that could see the Democrats regain some degree of power.