WASHINGTON (NYTIMES) - Defence Secretary James Mattis said on Wednesday (Nov 21) that United States troops stationed at the south-west border would not be armed with guns to confront incoming migrants, despite a White House directive that aims to protect border security officials by pairing them with military forces.

The White House memo seeks to expand the mission of the troops at the border to also include duties such as crowd control and temporary detention. But Mr Mattis said it left the final decision on what US soldiers and Marines could - and could not - do to the Defence Secretary.

Mr Mattis mentioned one possible instance in which troops might act: defending a border agent who was hit by a rock, and detaining the migrant who threw it. But asked whether such a situation might call for the US soldier to be armed, the Defence Secretary unequivocally said it would not.

"No," he said. "Not with a firearm."

Sent to the Pentagon on Tuesday night, the memo was signed by White House chief of staff John Kelly, not President Donald Trump. It is the latest in a series of White House directives to prod active-duty troops towards a more aggressive role at the border against an approaching caravan of thousands of migrants from Central America.

The Pentagon has resisted some of those efforts by raising the spectre of the Posse Comitatus Act, a law that dates back to the Reconstruction era, which bars US forces from engaging in law enforcement activities within the borders of the US.

US troops may be outfitted with riot gear for the border mission, Defence Department officials said.

Still, "we are not doing law enforcement", Mr Mattis insisted. "There is no arrest authority under Posse Comitatus for US federal troops."

The White House push for a more aggressive military role was undercut by an internal Homeland Security Department document, dated last Saturday, that described the probability of US border guards facing violence at the south-west border as "minimal".

The border intelligence analysis contrasted with White House statements stoking fear about migrants overrunning official ports of entry and attacking border guards to enter the US.