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UN human rights experts have expressed "legitimate concerns" about US juries failing to charge policemen involved in the deaths of two black civilians.

It is part of a broader "pattern of impunity" concerning minority victims, the UN said in a statement.

Thousands of people have taken to the streets in protest over the deaths of two black men at the hands of white officers in recent months.

Grand juries in Missouri and New York failed to charge either officer.

"I am concerned by the grand juries' decisions and the apparent conflicting evidence that exists relating to both incidents," UN Special Rapporteur on minority issues, Rita Izsak, said in a statement.

A trial process would ensure the evidence is considered in detail, she said.

"The decisions leave many with legitimate concerns relating to a pattern of impunity when the victims of excessive use of force come from African-American or other minority communities."

Media playback is unsupported on your device Media caption Watch: How the protests unfolded in a number of US cities

Human rights expert Mireille Fanon-Mendes-France, who currently heads the UN Working Group of Experts on People of African Descent, said the cases add to existing concerns.

She pointed to "longstanding prevalence of racial discrimination faced by African-Americans, particularly in relation to access to justice and discriminatory police practices".

The UN findings come amid ongoing protests over the death of Eric Garner, a black man held in an apparent chokehold by a white New York police officer.

Image copyright AFP Image caption Politicians have pledged to increase police training in the wake of the incidents

Image copyright Reuters Image caption Protesters have demanded justice following the death of a black man at the hands of a white police officer

Protesters have taken to major city streets, disrupting traffic and holding sit-ins after a grand jury last week decided not to press charges against the officer involved.

Civil rights activists are pinning their hopes on a federal investigation into the case.

After two nights of protests in New York, the city is preparing for the funeral on Friday of a man apparently inadvertently shot by a policeman in a dark stairwell.

Akai Gurley was unarmed when he was killed and his parents are demanding justice.

Akai Gurley

Image copyright Reuters

The 28-year-old father (daughter Akaila above) was killed in November in a Brooklyn apartment building as he walked with his girlfriend

he was shot by a rookie New York police officer

New York Police Commissioner William Bratton later said Gurley was a totally innocent victim

Gurley's family have demanded justice in the incident

On Friday, the Brooklyn district attorney's office said a grand jury will consider bringing charges in the case.

Following the outcry over the Garner case, the mayor of New York, Bill de Blasio ordered the city's 22,000-strong police force to be retrained in how to better communicate and remain calm when making arrests, and they will be fitted with body cameras.

13% of the US population is black 28% of suspects arrested in the US in 2010 were black

32% of people killed from 2003 to 2009 in arrest-related incidents were black

42% of inmates on death row in 2012 were black

US President Barack Obama welcomed the mayor's commitment to act, and said "too many Americans feel deep unfairness" in how the laws are applied.

The latest wave of race-related unrest began last week over the decision not to indict another white police officer who had shot dead a young black man, Michael Brown, in Ferguson, Missouri.

Elsewhere in the US several other racially sensitive cases appeared this week:

in Phoenix, Arizona, protesters demonstrated after a white police officer shot dead a black suspected drug-trafficker, after apparently mistaking a pill bottle in his pocket for a gun on Tuesday

in South Carolina, white ex-police chief Richards Combs was charged with murder over the 2011 shooting of an unarmed black man, Bernard Bailey

The justice department and the city of Cleveland, Ohio, agreed to overhaul city police after federal investigators found frequent use of excessive force caused deep mistrust, especially among black people.

On Thursday night in New York, protesters carried coffins across the Brooklyn Bridge and marched in several groups through Manhattan, carrying banners saying "Racism kills" and "This stops today".

Image copyright Reuters Image caption Streets in New York were shut down by rolling protests

Image copyright Getty Images Image caption Protests popped up across the US, including in Oakland, California

"People are sick and tired of the systemic problems of racism in this country," said one protester, Jason Pollock. "We are out here in the streets to say that black life matters."

Smaller-scale protests were held in cities including Chicago, Washington, Denver, and Boston. In Minneapolis, some protesters blocked traffic by marching or lying in the middle of a highway.

Activists have called for another march in Washington on 13 December, followed by a summit on civil rights.