US President Barack Obama

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.

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."

Ali died last Friday at the age of 74 in hospital in Phoenix, Arizona.

World leaders will be among thousands attending Friday's procession and memorial service in Louisville, Kentucky, where Ali was born.

The White House said Mr Obama and his wife Michelle will be at daughter Malia's high school graduation.

They will send a letter to the Ali family with White House senior adviser Valerie Jarrett, who knew Ali.

Bob Gunnell, a spokesman for the Ali family, said Mr Obama and Ali's widow Lonnie had spoken by telephone.