US police and security forces must not use disproportionate force against protesters and journalists in US cities including Portland or detain them unlawfully, the United Nations human rights office has said.

"There have been reports that peaceful protesters have been detained by unidentified police officers and that is a worry because it may place those detained outside the protection of the law and may give rise to arbitrary detention and other human rights violations," Liz Throssell, UN human rights spokeswoman, told a Geneva news briefing.

"The authorities should ensure that federal and local security forces deployed are properly and clearly identified and would use force only when necessary, proportionately and in accordance with international standards," she said.

The US Justice Department said yesterday it would investigate the use of force by federal agents against protesters in Portland after another night of unrest in which Mayor Ted Wheeler was tear-gassed.

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Video footage showed Mr Wheeler wearing goggles and a face mask surrounded by a large crowd. He held his nose and closed his eyes amid clouds of gas and fireworks exploding nearby.

Portland Mayor Ted Wheeler reacts after being exposed to tear gas fired by federal officers 

"This is an egregious overreaction on the part of the federal officers," Wheeler told The New York Times.

"This is flat-out urban warfare."

He said he had seen nothing that justified the police response.

"I'm not afraid but I am pissed off," he said.

The Justice Department's move comes following outrage by members of Congress, as well as rights activists and the public, over daily violent clashes in Portland between federal forces and demonstrators protesting against racism and police brutality.

Inspector General Michael Horowitz said his office has opened an investigation into the civil unrest, which escalated in the past week following reports of camouflaged federal agents using force and snatching people from the streets of the western city and putting them into unmarked cars.

A separate review is being conducted over a crackdown against protesters in the US capital's Lafayette Square on 1 June near the White House, ahead of a photo op by President Donald Trump in front of a church.

The Department of Homeland Security, whose law enforcement agents have come under fierce criticism for their behaviour in Portland, said its internal watchdog was also conducting a probe.

Mr Horowitz said investigators will "examine the DOJ's and its law enforcement components' roles and responsibilities in responding to protest activity and civil unrest in Washington, DC, and in Portland, Oregon over the prior two months.

"The review will include examining the training and instruction that was provided to the DOJ law enforcement personnel; compliance with applicable identification requirements, rules of engagement, and legal authorities; and adherence to DOJ policies regarding the use of less-lethal munitions, chemical agents, and other uses of force."

Mr Horowitz said his office will coordinate its probe with the Department of Homeland Security's watchdog.

The protests were initially sparked by the killing of George Floyd, an African American man who died at the hands of Minnesota police on 25 May. Similar demonstrations have taken place across the country.

Protesters use umbrellas as shields against teargas

US President Donald Trump has justified the deployment of the federal officers, saying they were needed to protect federal property and to restore order.

He has described the protesters as "anarchists and agitators" and has vowed to send federal officers to more US cities.