A US judge has ruled grand jury materials in Ghislaine Maxwell's sex-trafficking case will remain sealed, saying that making them public "would not reveal new information of any consequence".

The justice department had asked Judge Paul Engelmayer to unseal the documents, in an effort to assauge anger among President Donald Trump's supporters over its decision not to release all federal files on Maxwell's associate, deceased sex offender Jeffrey Epstein.

Maxwell, currently serving a 20-year prison sentence for crimes tied to Epstein, opposed unsealing the documents.

The judge wrote it was important to protect the secrecy of grand juries, who decide whether to indict people accused of crimes.

There are special circumstances where that secrecy is broken, Judge Engelmayer wrote in his 31-page decision.