Two US prisoners who have been held in solitary confinement for nearly 40 years should have their isolation ended immediately, Amnesty International said Tuesday.

Albert Woodfox, 64, and Herman Wallace, 69, have been held in solitary at Louisiana State Penitentiary ever since they were convicted of murdering a prison guard in 1972, the London-based human rights group said.

Their four-decade ordeal "is cruel and inhumane and a violation of the US's obligations under international law," said Guadalupe Marengo, Amnesty's Americas deputy director.

"We are not aware of any other case in the United States where individuals have been subjected to such restricted human contact for such a prolonged period of time."

The pair are suing the Louisiana authorities claiming that their prolonged isolation is "cruel and unusual punishment" and so violates the US constitution.

"The treatment of these men by the state of Louisiana is a clear breach of US commitment to human rights," said Marengo.

"Their cases should be reviewed as a matter of urgency, and while that takes place authorities must ensure that their treatment complies with international standards for the humane treatment of prisoners."

Amnesty said the men were confined to their cells, measuring two metres (6.5 feet) by three metres, for 23 hours a day, and have never been allowed to work or have access to education.