CHICAGO (AFP)

US President Donald Trump authorised deployment of troops to Chicago after a federal agent shot an allegedly armed motorist there on Saturday, while a judge blocked the Republican leader's attempt to send the military into Portland, another Democratic-run city.

"President Trump has authorised 300 national guardsmen to protect federal officers and assets" in Chicago, White House spokeswoman Abigail Jackson said in a statement, after weeks of the Republican threats to send troops to the Midwestern city over the wishes of local leaders.

"President Trump will not turn a blind eye to the lawlessness plaguing American cities."

Illinois Senator Dick Durbin called the move a "shameful chapter in our nation's history," adding that the "President is not intent on fighting crime. He is intent on spreading fear."

Chicago and Portland are the latest flashpoints in the Trump administration's rollout of anti-immigration and anti-crime raids, following the deployment of troops to Los Angeles and Washington earlier this year.

The raids have seen groups of masked, armed men in unmarked cars and armoured vehicles target residential neighborhoods and businesses.

Trump has repeatedly called Portland "war-ravaged" and riddled with violent crime, but in Saturday's court order, US District Judge Karin Immergut wrote "the President's determination was simply untethered to the facts."

Although the city has seen scattered attacks on federal officers and property, the Trump administration failed to demonstrate "that those episodes of violence were part of an organised attempt to overthrow the government as a whole," Immergut wrote in granting a temporary restraining order.

Earlier Saturday, a federal officer in Chicago shot a motorist after law enforcement agents were "boxed in by 10 cars," the Department of Homeland Security (DHS) said.

Trump's "Operation Midway Blitz" was launched in Chicago last month, and Saturday's shooting is not the first time federal agents have opened fire. Department of Homeland Security officials said ICE officers shot and killed 38-year-old immigrant Silverio Villegas Gozalez during a traffic stop on September 12.