PRESIDENT Donald Trump has signed a proclamation ordering the deployment of the National Guard to the US-Mexico border in a bid to reduce illegal immigration.

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

It came hours after he pledged “strong action” on immigration and a day after he said he wanted to use the military to secure the southern border until his stalled border wall is erected.

Trump has been frustrated by slow action on building his “big, beautiful wall” along the Mexican border, as well as a recent uptick in illegal border crossings.

Federal law prohibits the use of active-duty service members for law enforcement inside the US, unless specifically authorised by Congress. But over the past 12 years, presidents have twice sent National Guard troops to the border to bolster security and assist with surveillance and other support.

One congressional aide said that politicians anticipate 300 to 1200 troops will be deployed and that the cost was expected to be at least $60 million to $120m a year. In Mexico, senators urged President Enrique Pena Nieto to temporarily suspend co-operation with the US on immigration and security issues.