After enduring more than four decades in solitary confinement and lengthy court battles to prove his innocence, Albert Woodfox was told by a US appeals court Tuesday he must wait in prison a little longer.

A federal judge had ordered his unconditional release in a strongly-worded ruling Monday that barred any further trial on the murder charges.

But Louisiana's attorney general filed an appeal and won a temporary stay blocking Woodfox's release until at least Friday.

Woodfox, 68, has served more years in solitary than any other US prisoner: 43.

A member of the radical Black Panther movement, Woodfox was among three men who were convicted of the murder of a guard during a 1972 prison riot.

The men became known as the "Angola Three" as their years in solitary confinement at the notorious Louisiana State Penitentiary at Angola drew attention to their avowals of innocence.

Woodfox is the last of them to remain behind bars.

In November, the US Court of Appeals for the Fifth Circuit overturned Woodfox's conviction, but he was charged again at state level in February.

Woodfox has long maintained his innocence, namely citing a witness who said he was not involved in the murder, as well as scientific review of evidence at the scene that exculpated him and a polygraph test that found he had truthfully denied his involvement.

"Evidence supporting claims of Mr Woodfox's innocence gives this Court even more reason to question his two previously overturned convictions," US District Judge James Brady wrote in ordering his release.

Brady listed five "exceptional circumstances" for Woodfox to be released unconditionally and immediately, rather than the usual conditional release pending trial.

In addition to Woodfox's age and "poor health," the judge also listed his "lack of confidence in the state to provide a fair third trial, the prejudice done onto Mr Woodfox by spending over 40 years in solitary confinement, and finally the very fact that Mr Woodfox has already been tried twice and would otherwise face his third trial for a crime that occurred over 40 years ago."