US President Barack Obama on Wednesday called the mayors of Tulsa and Charlotte, where the shootings of two black men by police have reignited summer-long racial unrest.

"Both mayors provided the President with an update on the situation on the ground and the protests in their cities," a White House official said.

"The President and both mayors reiterated that any protests should be conducted in a peaceful manner and that local law enforcement should find ways to calmly and productively engage those protesting."