More than 12,000 inmates in California jails have missed nine consecutive meals over three days in a hunger strike against solitary confinement and prison authorities are now threatening to discipline them.

The nine-meal point is a critical benchmark that requires officials to recognise the action as a hunger strike.

About 30,000 prisoners across the state began refusing meals on Monday in support of inmates with ties to

gangs who are being held in solitary confinement at Pelican Bay State Prison. Some prisoners are also refusing to work and to attend classes.

"Participating in a mass disturbance and refusing to participate in a work assignment are violations of state law, and any participating inmates will receive disciplinary action," California prison officials said in a statement.

Joining in this current strike "can lead to loss of privileges, loss of credits," said Jeffrey Callison, a spokesman for the Department of Corrections and Rehabilitation. He did not say what types of inmate privileges could be taken away.

Concessions unlikely

California's prison chief, Jeffrey Beard, has said inmates are unlikely to be offered concessions and that there is already a scheme to reduce isolation sentences, some of which can last for decades.

The state locks 4,527 inmates in cells that are isolated from the general prison population because prison officials found that they had either committed crimes behind bars or are affiliated to gangs, a prison spokesman told reporters.

To call it a yard is to call your bathroom a yard when you open the window. Carol Strickman, prisoners' rights attorney

Corrections officials had said that more than 30,000 of California's 132,800 inmates began refusing food on Monday and continued to decline meals through the week, with nearly 29,000 participating on Wednesday.

On Thursday, officials said 12,421 prisoners in 24 state prisons and four out-of-state facilities had missed nine straight meals.

A spokesperson for the Department of Corrections and Rehabilitation said some prisoners in isolation had cellmates, that they were allowed exercise in a yard at least 11 hours a week and that they had access to a law library and cable TV.

Carol Strickman, a prisoners' rights attorney who represents some of the hunger strikers in a lawsuit against the state, said inmates were frustrated that after two widely publicised hunger strikes in 2011, the state had barely changed its procedures for solitary confinement.

Strickman said the yard was an unheated concrete room about the size of three parking spaces, with a ceiling made of clear plastic or glass that opens only partially. Inmates are also alone when they are in the space, she said.

"To call it a yard is to call your bathroom a yard when you open the window," Strickman said.