US Supreme Court Justice Ruth Bader Ginsburg apologised Thursday for publicly disparaging Donald Trump as a phoney with a big ego.

The left-leaning 83-year-old judge shocked the US political establishment with her recent remarks: on Monday she called Trump "a faker" who "says whatever comes into his head at the moment."

She also told The New York Times in an interview published last weekend that she "can't imagine" what the United States under Trump as president would be like.

Ginsburg issued a statement Thursday saying she was sorry.

"On reflection, my recent remarks in response to press inquiries were ill-advised and I regret making them," Ginsburg said.

"Judges should avoid commenting on a candidate for public office. In the future I will be more circumspect," she added.

In response to the criticism, Trump had suggested the judge was getting senile and should step down.

"Justice Ginsburg of the US Supreme Court has embarrassed all by making very dumb political statements about me. Her mind is shot - resign!" he tweeted.

Ginsburg drew widespread ire from Republicans for breaking with a code of conduct under which US judges are not supposed to publicly endorse or oppose a candidate for office.

The Washington Post and The New York Times both ran editorials saying she had erred in publicly criticising Trump.