US President Donald Trump will sign a proclamation ordering the deployment of the National Guard to help protect the border with Mexico, Homeland Security Secretary Kirstjen Nielsen says.

Troops may be heading to the border as early as Wednesday night, Ms Nielsen said, saying that the National Guard would support US Custom and Border Protection but would not be involved in enforcement.

Ms Nielsen spoke at a White House news briefing a day after Mr Trump sharpened his anti-immigration rhetoric by saying he wanted to deploy US military forces until his promised border wall is built.

“The president has directed that the Department of Defense and the Department of Homeland Security work together with our governors to deploy the National Guard to our southwest border to assist the Border Patrol,” Ms Nielsen said. “The president will be signing a proclamation to that effect today.” She said the administration had drafted legislation and would be asking congress to provide the legal authority and resources to address “this crisis at our borders”.

She did not give the number of the troops to be deployed or the cost of the operation.

Ms Nielsen said that despite steps taken by the administration, the levels of drug smuggling, illegal immigration and dangerous gang activity across the border were unacceptable.

Mr Trump met with Defence Secretary James Mattis, Attorney-General Jeff Sessions, Ms Nielsen and other officials to discuss border issues on Tuesday.

“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” US laws that left the southern border poorly protected.

On Wednesday, Mr Trump said in a tweet: “Our Border Laws are very weak while those of Mexico & Canada are very strong. Congress must change these Obama era, and other, laws NOW!”