Federal immigration agents shot dead a man in Minneapolis on Saturday, in the second fatal shooting of a civilian during the Trump administration's unprecedented operation in the city, sparking fresh protests and outrage from state officials.

The Department of Homeland Security (DHS) insisted the man killed Saturday, identified by local media as 37-year-old nurse Alex Pretti, "approached" agents with a pistol and then "violently resisted" being disarmed.

Crowds quickly gathered in the wake of the shooting, despite the sub-freezing temperatures and making state law enforcement unable to secure the scene, Minnesota Department of Public Safety Bob Jacobson told a briefing.

US President Donald Trump meanwhile ratcheted up his war of words with Minnesota leadership, accusing the Democratic leaders of "inciting insurrection" with their rhetoric.

Minnesota Representative Ilhan Omar called the shooting "an execution" and accused Trump of transforming Minneapolis into a "war zone."

Thousands of ICE agents have been deployed to the Democratic-led city, as Trump presses a sweeping campaign to deport undocumented migrants. (AFP)