A notorious US inmate has been killed in a jail yard riot.

Hugo Pinell, 71, who was the last member of the 'San Quentin Six,' was attacked by another inmate at maximum security California State Prison-Sacramento.

A skirmish reportedly broke out in the exercise yard on Wednesday (12 August) with 70 inmates caught in the melee. The riot was broken up by prison officers using pepper spray and other "non-lethal force", officials said.

11 inmates were taken to hospitals where they were treated for stab wounds while numerous prisoners suffered other injuries. No members of staff were injured during the prison brawl.

It is unclear if the violence was sparked by Pinell's death or if he died in the crush.

Pinell had been in custody since 1965. He was originally convicted of rape and sentenced to life with the possibility of parole. He was later convicted of the March 1971 murder of a prison guard and received another life sentence.

In August of that year, Pinell received a third life sentence after he and other inmates took part in an infamous San Quentin State Prison jailbreak attempt 44 years ago

He slit the throats of two officers during the attempt. Three guards, two inmate workers and escape ringleader George Jackson lost their lives during the failed prison break.

An investigation into how the incident unfolded is under way.

California State Prison-Sacramento houses some 2,300 inmates, most of whom are serving lengthy prison sentences.