US President Donald Trump has signed a proclamation today sending National Guard troops to the southern border to ramp up security efforts, after days of warnings from the President about the dangers of illegal migration to the border.

The National Guard will be sent to assist Border Patrol agents until legislation is passed to close "loopholes" that the Trump Administration says allows certain undocumented immigrants to stay in the country and the "catch-and-release" policy is ended, according to a source familiar with the expected White House proposal.

Catch and release is a practice of releasing undocumented immigrants from custody while they await their hearings.

The troops will be in a support role, doing tasks such as road development and intelligence gathering, according to the source.

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Trump says in a memorandum to his secretaries of defence and homeland security and to his attorney-general that the "situation at the border has now reached a point of crisis."

The document orders the Secretary of Defence to support the Department of Homeland Security in securing the southern border to stop the flow of drugs and people. And it orders the agency heads to submit a report within 30 days outlining what other steps can be taken.

Trump says that "lawlessness" at the southern border is "fundamentally incompatible with the safety, security, and sovereignty of the American people." And he says his Administration "has no choice but to act."

Trump first floated the prospect of sending military personnel to the border yesterday, calling it a "big step" during a meeting with the leaders of three Baltic nations at the White House.

He has been briefed twice in recent days by senior Administration officials about the migration issue, and has "directed a vigorous administrative strategy to confront this threat and protect America's national security," according to White House press secretary Sarah Huckabee Sanders.

In this June 20, 2008 photo, members of the 200th Red Horse Air National Guard Civil Engineering Squadron from Ohio, work on building a road at the border in Nogales, Arizona. Photo / AP file
In this June 20, 2008 photo, members of the 200th Red Horse Air National Guard Civil Engineering Squadron from Ohio, work on building a road at the border in Nogales, Arizona. Photo / AP file

Earlier, Trump teased the announcement on Twitter: "Our Border Laws are very weak while those of Mexico & Canada are very strong. Congress must change these Obama era, and other, laws NOW! The Democrats stand in our way - they want people to pour into our country unchecked....CRIME! We will be taking strong action today."

Previous presidents, including Barack Obama and George W. Bush, have deployed National Guard personnel to provide backup to Border Patrol agents amid concerns about security and drug trafficking.

In recent years, however, the number of people crossing illegally into the country has plummeted and is at its lowest level since 1971.

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