The reported preparations come days after President Donald Trump threatened to invoke the Insurrection Act, which enables use of the military to suppress "armed rebellion" or "domestic violence" -- although a day later he said there was no immediate need for it.

The troop deployment plans were first reported by the ABC. The news network cited two unnamed US defense officials as saying 1,500 active-duty Army paratroopers based in Alaska have been put on alert, but the president has not made a final decision on the deployment.

The Pentagon and the White House did not immediately respond to AFP requests for comment on Sunday.

The Insurrection Act was last invoked in the United States more than 30 years ago, and any move by the federal government to deploy the military would inflame the standoff between the White House and local authorities in Minnesota.

Crowds of protesters have clashed with immigration officers in the midwestern state's main city of Minneapolis, particularly since the fatal shooting of a US woman by a federal agent on January 7.

On Saturday, the Minnesota Department of Public Safety said that at the direction of Governor Tim Walz the state's national guard has been mobilized to support local law enforcement and emergency management agencies. As governor, Walz has the authority to deploy the state's US military reserve during emergencies.

Minneapolis Mayor Jacob Frey on Sunday described the 3,000 federal agents from Immigration and Customs Enforcement (ICE) and border control waging Trump's crackdown on illegal immigration as an "occupying force that has quite literally invaded our city."

He told CBS's "Face the Nation" that the federal agents outnumbered the 600-strong city police force by five to one. He also voiced concern about reports of 1,500 federal troops readying to join them.

"This is not about safety. What this is about is coming into our city by the thousands and terrorizing people simply because they're Latino or Somali, and yeah, people in Minneapolis are speaking up," Frey said.

Secretary of Homeland Security Kristi Noem said that the crackdown would continue "until we are sure that all the dangerous people are picked up, brought to justice and then deported back to their home countries."

The Pentagon deployed some 700 US Marines in Los Angeles in June and July in response to angry protests over aggressive immigration enforcement operations underway there.

Trump also threatened at that time to invoke the Insurrection Act, but ultimately did not, and the soldiers' role was limited to guarding two federal properties in the greater Los Angeles area.