Geneva, Switzerland - The United Nations rights chief voiced astonishment Friday at the "now-routine abuse" of migrants by US authorities, urging Washington to end practices "tearing apart families."

Volker Turk called on the US to ensure migration policies and enforcement practices respect human dignity and due process and slammed the "dehumanising portrayal and harmful treatment of migrants and refugees."

"I am astounded by the now-routine abuse and denigration of migrants and refugees," he said in a statement.

"Where is the concern for their dignity, and our common humanity?"

Thousands of Immigration and Customs Enforcement agents have been deployed to Democratic-led Minneapolis, as the Republican administration of President Donald Trump accelerates its mass deportation campaign.

On Thursday, Democrats and local officials in Minneapolis expressed outrage at the detention of a five-year-old boy in the massive immigration crackdown.

Protests have continued in Minneapolis since federal agents shot and killed US citizen Renee Good there on January 7, even as Trump and his officials quickly defended the agent's actions as legitimate self-defense.

The UN High Commissioner for Human Rights deplored the use of large-scale enforcement operations by ICE and other agents who he said were repeatedly using apparently unnecessary or disproportionate force.

"Under international law, the intentional use of lethal force is only permissible as a measure of last resort against an individual representing an imminent threat to life," Turk stressed.

Turk warned that numerous US migration policies were resulting in arbitrary and unlawful arrests and detentions, as well as flawed removal decisions.

"Individuals are being surveilled and detained, sometimes violently, including at hospitals, churches, mosques, courthouses, markets, schools, and even within their own homes, often solely on mere suspicion of being undocumented migrants," Turk said.

"Those who dare to speak up or protest peacefully against heavy-handed immigration raids are vilified and threatened by officials, and on occasion subjected to arbitrary violence themselves."

He said many arrests, detentions, and expulsions occur without efforts to assess and maintain family unity, exposing especially children to risks of severe and long-term harm.

"I call on the administration to end practices that are tearing apart families," he said.

Turk acknowledged that "states have the authority to establish their national migration policies," but "this needs to be carried out in full accordance with the law."

He also expressed deep concern about the dehumanizing narratives frequently used to describe migrants and refugees, especially in the US that had been shaped so "profoundly by the contributions of migrants."

"Demonising migrants and refugees collectively as criminals, threats, or burdens on society – based on their origin, nationality or migration status – is inhuman, wrong, and it goes against the very fabric and foundations of the nation."

Turk urged "leaders at all levels in the US to halt the use of scapegoating tactics that seek to distract and divide, and which increase the exposure of migrants and refugees to xenophobic hostility and abuse."

The UN rights chief also called for a "independent and transparent investigation" into deaths in ICE custody.

At least 30 such deaths were reported last year, and a further six have been reported so far this year, he pointed out.