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President Donald Trump says his administration will send "a surge" of federal agents to US cities in an anti-crime crackdown, Operation Legend.

Speaking from the White House, Mr Trump said he had "no choice" but to deploy hundreds of federal agents to Chicago.

The Republican is also sending federal officers to two other Democratic-run cities, Albuquerque and Kansas City.

Federal officers are already in Portland, Oregon, where the mayor says they are making ongoing protests worse.

US Attorney General William Barr, who joined Mr Trump at Wednesday's announcement, said they had sent about 200 federal agents to Kansas City, Missouri, would send a "comparable" number to Chicago, and about 35 others to Albuquerque, New Mexico.

President Trump is adopting a law-and-order posture as he faces a tough re-election campaign in November.

There have been protests - sometimes descending into civil disorder - in scores of US cities since May, following the death of an unarmed black man, George Floyd, in police custody in Minneapolis, Minnesota.

Meanwhile, gun violence has spiked in cities including New York City, Philadelphia, Los Angeles, Chicago and Milwaukee.

On Tuesday night, federal agents fired tear gas, pepper balls and flashbangs at demonstrators in central Portland, which has seen 54 consecutive night of protests. 

The officers quickly used crowd-control munitions in an attempt to disperse hundreds of people gathered outside a federal court.

The agents have been accused of driving in unmarked vehicles around Oregon's biggest city while wearing military fatigues and arbitrarily arresting a handful of demonstrators.

Chicago Mayor Lori Lightfoot confirmed on Tuesday that federal agents would be deployed to her city to beef up local police.

"We welcome actual partnership, but we do not welcome dictatorship," Ms Lightfoot told a news conference, warning that the federal officers should not try the same tactics they have used in Portland.

Mr Trump often highlights Chicago's gun violence epidemic and he did so again on Wednesday.

"This rampage of violence shocks the conscience of our nation," he said.

In the latest incident in Chicago, at least 15 people were shot outside a funeral home in a suspected gang-related attack.

Federal officers were deployed to Kansas City following the death of a four-year-old boy, LeGend Taliferro, who was shot in the face while sleeping in his family home in June.

The federal law enforcement operation is named after the boy, Mr Trump said. LeGend's mother joined the president at the announcement.

"No mother should ever have to cradle her dead child in her arms simply because politicians refused to do what is necessary to secure their neighbourhood and to secure their city," Mr Trump said.