As the White House worked to "de-escalate" tensions in Minneapolis on Tuesday, a man sprayed Minnesota Congresswoman Ilhan Omar with a syringe of unknown liquid at a town hall meeting, where she called for curbing the Trump administration's anti-immigration crackdown.

The White House is also evaluating whether the Customs and Border Protection (CBP) agents who gunned down a nurse failed to follow "clear guidance" to "create a physical barrier between the arrest teams and the disruptors," deputy chief of staff Stephen Miller told reporters.

"We are evaluating why the CBP team may not have been following that protocol," Miller said in a statement.

At the town hall Tuesday night, Omar had just spoken about the abolition of Immigration and Customs Enforcement (ICE) and demanded that Department of Homeland Security Secretary Kristi Noem "resign or face impeachment," when a man sprang up from the front row, made a remark and sprayed the lawmaker, as security leapt to grab him.

Omar raised a fist and stepped toward the attacker before returning to the podium to say: "Here's the reality that people like this ugly man don't understand: We are Minnesota strong. And we will stay resilient in the face of whatever they might throw on us."

Omar is a frequent target of US President Donald Trump -- who continues to defend Noem despite her oversight of ICE agents who shot and killed two American citizens this month, saying Noem would not step down and was doing a "very good job."

But after days of protests following 37-year-old Alex Pretti's death, Trump told Fox News "we're going to de-escalate a little bit," while adding that it was not a "pullback."

Trump also admitted that Gregory Bovino, a hardline Border Patrol commander who is now expected to leave Minneapolis, was "a pretty out-there kind of a guy" whose presence may not have helped the situation, and sent top US border security official Tom Homan to meet with officials in the city.

Trump told reporters that he rejected the "assassin" label used by a top aide to describe the 37-year-old nurse, adding that he wants "a very honorable and honest investigation."

But he criticised Pretti for carrying the licensed firearm that was taken off him before he was shot.

"I don't like that he had a gun, I don't like that he had two fully loaded magazines," the president said. (AFP)