US National Guard to be deployed to Mexico border 'immediately' as Donald Trump warns of 'crisis'

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.

She declined to say how many personnel would be needed or how much the operation would cost, but she insisted: "It will be as many as is needed to fill the gaps that we have today."

Mr Trump first announced his plan to send the military to the border during a meeting with Baltic leaders on Tuesday.

Mr Trump's plans were hailed by Republican Arizona Governor Doug Ducey, who tweeted: "Arizona welcomes the deployment of National Guard to the border."

The California National Guard will promptly review the president's request "to determine how best we can assist our federal partners," said a spokesman for the state's unit in a statement. He added he was speaking for Governor Jerry Brown, a Democrat.

Texas Governor Greg Abbott, a Republican, said in a statement: "Today's action by the Trump administration reinforces Texas' longstanding commitment to secure our southern border and uphold the Rule of Law, and I welcome the support."

President Barack Obama and President George W. Bush both took similar temporary measures.

Mr Trump has expressed frustration in recent days over what he called a "caravan" of more than 1,000 migrants heading through Mexico to the US border.

He has also been frustrated by lack of funding from congress for his border wall.

"Until we can have a wall and proper security we're going to be guarding our border with the military," Mr Trump told reporters at the White House on Tuesday, lamenting what he called "horrible" U.S. laws that left the border poorly protected.

The caravan of Central American migrants on Wednesday scrapped plans to cross into the United States.

The US / Mexico Border, in pictures

Its leaders said most of the group - about 80 per cent - would now remain in Mexico, where authorities were working with individual migrants and families to get them temporary papers.

Irineo Mujica, head of migrant advocacy group People Without Borders, said: "All they want is a place to live in peace, where they can work without having guns pointed at them, without being forced to join a gang."