WASHINGTON, April 4 (Xinhua) -- U.S. President Donald Trump signed a proclamation on Wednesday, authorizing the deployment of the National Guard to the border with Mexico, citing a crisis situation.

"The situation at the border has now reached a point of crisis," Trump said in a memorandum to his secretaries of defense and homeland security and to his attorney general.

"The lawlessness that continues at our southern border is fundamentally incompatible with the safety, security, and sovereignty of the American people," he said. "My administration has no choice but to act."

The memo asks Defense Secretary James Mattis to support the Department of Homeland Security in securing the southern border and stop the flow of drugs and criminals.

It gives 30 days to the agency heads to submit a report on their findings, along with an action plan.

Early on Wednesday, Trump had forewarned in a tweet that the administration would be "taking strong action today" on immigration.

During a White House news briefing on Wednesday, Homeland Security Secretary Kirstjen Nielsen said troops could be sent to the border as early as Wednesday night. She said the National Guard would support U.S. Custom and Border Protection but would not be involved in enforcement.

According to U.S. federal laws, the military are prohibited from being used for law enforcement. Troops can however play support roles like in training, construction and intelligence gathering.

Nielsen did not detail the number of troops to be deployed, where they would go and how long they would stay there, or the cost of the operation. All she said was, "It will be strong, it will be as many as needed to fill the gaps."

The announcement came after Trump publicly fumed for days over the immigration policy and border security situation, due to what he called "weak" border laws. He has repeatedly called out Mexico over border security and stressed the need for a wall on the southern border.

The 1.3-trillion-U.S.-dollar spending bill signed into law by him last month grants only 1.6 billion dollars for border security measures, far short of what the administration sought.

Trump has proposed using military funding to build the wall but it's likely to face both political and legal challenges.

Though sending the National Guard to the border is not unprecedented, similar moves by Trump's predecessors were criticized for their high cost and limited effectiveness.