Ms Manning, 31, was last month sent to jail indefinitely after she refused to take part in a department of justice investigation into WikiLeaks, and told judges she would accept “whatever you bring upon me”. The investigation is using secret hearings involving a grand jury.
“I will not participate in a secret process that I morally object to, particularly one that has been used to entrap and persecute activists for protected political speech,” she said, as the court ordered her to be held until she agreed to cooperate, or else the grand jury process was completed.
On Monday, an appeals court denied the whistleblower’s request to be released on bail.
Her lawyers had argued that trying to force her to give evidence against Mr Assange, who earlier this month was arrested in London and charged by the US of conspiring with Ms Manning to try and hack a US computer, breached her rights under the first, fourth and sixth amendments of the US constitution.
Her lawyers said that among other things, the lower court had failed to address her concerns that the government was abusing the grand jury process so it could preview or undermine her testimony as a potential defence witness at a trial.
Her lawyers have also argued that the courtroom was improperly sealed during substantial portions of the hearing.
But a three-judge panel of the US Court of Appeals for the Fourth Circuit did not agree.
“The court finds no error in the district court’s rulings and affirms its finding of civil contempt,” they wrote.
There was no immediate response from Ms Manning or her lawyers.
Ms Manning served seven years in jail between 2010–2017 after being charged with more than a dozen crimes, after leaking 750,000 classified, or unclassified but sensitive documents to WikiLeaks.
Among the information, was video footage showing US military helicopter attack on civilians in Baghdad that killed more than a dozen people.
Mr Assange was arrested on April 11 at the Ecuadorean Embassy in London, after US prosecutors in the Eastern District of Virginia unsealed a criminal case against him, alleging he conspired with Ms Manning to commit computer intrusion.
The justice department said Mr Assange was arrested under an extradition treaty between the United States and Britain.