MINNEAPOLIS — United States immigration agents may have breached "protocol" in Minneapolis before shooting dead a nurse during protests, a senior White House aide said on Tuesday, as the Donald Trump administration suggested it would "de-escalate a little bit" in the city.

The comments came the same day a man sprayed Minnesota Congresswoman Ilhan Omar with a syringe of unknown liquid at a Minneapolis town hall meeting, where she called for curbing the administration's immigration crackdown.

White House deputy chief of staff Stephen Miller told Agence France-Presse that the administration is evaluating whether the Customs and Border Protection agents who gunned down 37-year-old Alex Pretti on Saturday failed to follow "clear guidance" to "create a physical barrier between the arrest teams and the disrupters".

"We are evaluating why the CBP team may not have been following that protocol," Miller said in a statement, though the White House later said Miller was referring to the "general guidance" to immigration agents in Minnesota, not the specific incident involving Pretti.

Trump, meanwhile, told Fox News that his administration was "going to de-escalate a little bit" its operations in Minneapolis, adding that the plan was not a "pullback".

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

Trump also told reporters that he rejected the "assassin" label previously used by Miller to describe Pretti, adding he wants "a very honorable and honest investigation".

Still, tensions remained high in Minneapolis after Omar, a Democrat, was attacked by a man at a constituent town hall on Tuesday night.

Omar had just spoken about the abolition of Immigration and Customs Enforcement and demanded that Department of Homeland Security Secretary Kristi Noem "resign or face impeachment", when the man sprang from the front row, made a remark and sprayed the lawmaker with liquid from a syringe as security leaped to grab him.

After meeting with Homan on Tuesday, Minneapolis Mayor Jacob Frey said in a statement that they discussed the "serious negative effects this operation has had on Minneapolis".

The city "will not enforce federal immigration laws", he said.

Just weeks after federal immigration agents shot and killed US citizen Renee Good in Minneapolis, Pretti's death sparked national outrage and added to a litany of complaints of abusive tactics.

The killings capped months of escalating violence in which masked, unidentified and heavily armed Immigration and Customs Enforcement and Border Patrol agents have grabbed people they accuse of violations off the streets.

Despite multiple videos showing that Pretti posed no threat, Bovino and Noem initially claimed Pretti had the intent to kill federal agents, calling him a "domestic terrorist".

Agencies via Xinhua