National Guard troops could be at the US-Mexico border within hours after being ordered there by Donald Trump.

The Trump administration said on Wednesday night it was working with state governors to "immediately" deploy the National Guard to combat illegal immigration at the border.

Asserting the situation had reached "a point of crisis," President Donald Trump on Wednesday signed a proclamation ordering the deployment.

"The lawlessness that continues at our southern border is fundamentally incompatible with the safety, security, and sovereignty of the American people," Mr Trump wrote in a memo authorising the move, adding that his administration had "no choice but to act."

Kirstjen Nielsen, the homeland security secretary, earlier said the troops could begin heading to the border immediately, though other administration officials cautioned that details on troop levels, locations and timing were still being worked out.

Ms Nielsen said her department had developed a list of locations where it would like assistance on things like aerial surveillance and other support, and was discussing with the governors how to facilitate the plans.