US President Barack Obama has said all Americans should be concerned about the frequent police killings of black men.

His comments, as he arrived in Poland for a Nato summit, came after two more black men died in such incidents.

Philando Castile was shot dead during a traffic stop in Minnesota on Wednesday, while Alton Sterling was shot dead by police in Louisiana a day before.

On Thursday night, protests were held across the US over the police use of lethal force against African Americans.

Philando Castile was a school cafeteria supervisor

Demonstrations took place in several cities including, Dallas, New York, Chicago, Washington and the two cities that features in the latest incidents, Baton Rouge in Louisiana and St Paul in Minnesota.

Mr Obama said the US must say "we're better than this", and that its importance went beyond race.

"This is not just a black issue. It's not just a Hispanic issue. This is an American issue that we should all care about," he said.

"All fair-minded people should be concerned."

Pointing to statistics showing African-American citizens are far more likely to be shot by police by whites, Mr Obama called on law enforcement to root out internal bias.

"When incidents like this occur, there's a big chunk of our fellow citizenry that feels as if it's because of the colour of their skin, they are not being treated the same,'' he said. "And that hurts."

The latest incidents follow a long line of controversial deaths of African-Americans at the hands of the police that has ignited a national debate about the use of lethal force.

Some of the people bringing bouquets to the site of the shooting are too emotional to talk. "I'm just numb and sad," mumbles a middle-aged black woman.

An elderly white woman, Diana, has driven 40 miles to this quiet middle-class suburb to pay her respects. She was part of the civil rights movement that protested against discrimination, she says, "and it's still going on".

Joe, an elderly man passing round blueberries to protesters gathered outside the governor's mansion, agrees: "It's an indictment of my generation of white people."

The rally is multi-racial and peaceful but black anger is visceral. "He (Castile) lost his life for a broken tail light," spits out one speaker.

Diamond Reynolds made an emotional statement outside the governor's residence in St Paul

"Use your white privilege to help us," admonishes another. A pastor and Iraqi war vet, Thomas, offers this bleak view of the police: "This is the same as a combat zone," he says. "If black people get pulled over we need to position ourselves as prisoners of war and survive the encounter."

Minnesota Governor Mark Dayton, who has requested a federal investigation into the shooting in St Paul, , Minnesota, said he didn't think Philando Castile would have been shot if he had been white.

"Would this have happened if the driver and the passengers were white? I don't think it would have," he told reporters.

"This kind of racism exists and it's incumbent on all of us to vow and ensure that it doesn't continue to happen."

The national debate has been stoked by videos of both incidents that quickly went viral on social media.

Philando Castile's girlfriend live-streamed the aftermath of the shooting in St Paul showing him covered in blood as an officer pointed a gun at him.

Diamond "Lavish" Reynolds was heard telling the police officer that her boyfriend had been reaching for his wallet, as he had been instructed to do.

"You shot four bullets into him, sir. He was just getting his licence and registration, sir," she says in the video.

The officer can be heard shouting: "I told him not to reach for it. I told him to get his hand out."

An emotional Ms Reynolds joined protesters outside Governor Dayton's official residence in St Paul, saying that she had filmed the incident so "the world knows that these police are not here to protect and serve us, they are here to assassinate us".

Leading black celebrities also joined the calls for action. Singer Beyonce published a statement on her website, calling on people to "take a stand and demand that they stop killing us".

Mr Castile, 32, worked as a cafeteria supervisor at a Montessori school. His cousin Antonio Johnson told the Star Tribune newspaper he was "immediately criminally profiled" because he was black.

Hundreds of people also gathered for a second night of protests in Baton Rouge, Louisiana at the shop where Alton Sterling, a 37-year-old father of five, was killed on Tuesday.

A second piece of video from Baton Rouge emerged on Wednesday appearing to show Mr Sterling being held down and then shot several times, although some shots are heard when the camera moves away from the confrontation.

Seconds later, one of the officers is seen removing an object from the man's trousers as he lies on the ground with blood on his chest.

A witness said he saw officers take a gun from Mr Sterling's pocket after the shooting, but police have not commented on this.

The officers involved, Blane Salamoni and Howie Lake II, have been put on administrative leave and the US department of justice has launched a civil rights investigation.

The officer involved in the St Paul shooting has also been placed on administrative leave.

Police killings that scar the US

Walter Scott - unarmed and shot in the back as he ran away from an officer in North Charleston, South Carolina, in April 2015. Former officer Michael Slager facing murder charge

Laquan McDonald - 17-year-old was holding a knife but appeared to be moving away from police in Chicago when shot 16 times in 2014. Officer Jason Van Dyke denies murder charge

Michael Brown - 18-year-old shot at least seven times in Ferguson, Missouri, in August 2014, sparking nationwide protests. Officer Darren Wilson cleared of wrongdoing

Eric Garner - died after being placed in a chokehold by New York police while selling cigarettes in July 2014. Grand jury decides against charges, police disciplinary action taken against supervising officer Sgt Kizzy Adonis