An US man kept in solitary confinement for 43 years has been released after his murder conviction was overturned on appeal.

Albert Woodfox was convicted twice for killing a prison guard in 1972 but a judge ordered his release last summer.

The former Black Panther was kept behind bars by a federal appeals court as the state of Louisiana, where he was imprisoned, lodged a challenge against his release.

His release on Friday came after the state dropped the threat of a third murder charge in exchange for Woodfox pleading no contest to lesser charges.

'Cruel and inhumane'

"Although I was looking forward to proving my innocence at a new trial, concerns about my health and my age have caused me to resolve this case now and obtain my release," Woodfox said in a statement after he was freed.

Woodfox's supporters have insisted his imprisonment and detention in solitary confinement were politically motivated.

His legal team has called on authorities to end the use of solitary confinement as a punishment, describing the practice as "cruel and inhumane".

The activist was one of three men known as the "Angola Three", alongside Robert King and Herman Wallace, who supporters say were kept in isolation because they fought for better prison conditions.

King was released in 2001 after his conviction was overturned and Wallace died in 2013 just two days after his release.