The Chief Justice of the United States, John Roberts, has told President Donald Trump that there are no Obama judges or Trump judges and instead what he calls "an extraordinary group of dedicated judges".

That's a view countered by the president himself, who says there are indeed "Obama judges" as he originally asserted in remarks critical of a federal judge who rejected his migrant asylum policy.

The back-and-forth represents an extraordinary public dispute over judicial independence.

It's highly unusual for a president to single out judges for personal criticism.

But it's unprecedented in modern history for a chief justice to challenge a president's comments.

Roberts and several of his Supreme Court colleagues have rebutted perceptions of the court as a political institution divided between conservative Republicans and liberal Democrats.