Hundreds of inmates across California prisons are continuing their hunger strike, one month after starting protests to highlight what they say are inhumane conditions in solitary confinement.

Sceptical prison officials have dismissed the action as a ploy by gang leaders to regain power.

Four inmates spearheaded the strike that began on July 8 and drew participation from an estimated 29,000 prisoners.

That number has since dwindled to 346 inmates in nine prisons, according to state corrections officers.

Inmates hope the hunger strike, the largest of its kind in California history, will spotlight what they term as "cruel and inhumane" conditions in cells known as Secure Housing Units, where hundreds of inmates have been held in isolation for more than a decade, many with limited human contact.

Prisoner demands include limits on the time inmates can spend in isolation, more family visits and increased access to rehabilitation and nutritious food.

The complaints form the centrepiece of demands from prisoners and their supporters calling for general reforms to the penal system, including reduced crowding and adequate medical care.

'Cruel and ineffective'

Rights groups such as the American Civil Liberties Union (ACLU) have backed prisoner demands, saying solitary confinement is cruel and ineffective and noting that prisoners are deprived of human contact for 22-and-half hours a day.

Most of California's 10,000 inmates in solitary confinement, the ACLU adds, are either mentally ill or secluded for minor infractions.

Mentally healthy inmates, psychologists argue, are pushed to the edge by long stays in isolated cells.

Juan Méndez, the UN special rapporteur on torture, has called on all countries to ban solitary confinement except in very exceptional cases.

"Considering the severe mental pain or suffering solitary confinement may cause, it can amount to torture or cruel, inhuman or degrading treatment or punishment when used as a punishment, during pre-trial detention, indefinitely or for a prolonged period, for persons with mental disabilities or juveniles" Méndez warned the UN General Assembly in 2011.

But California prison officials tell a different story.

They say gang leaders from the Aryan Brotherhood, the Mexican Mafia, Nuestra Familia and the Black Guerrilla Family, organised the strike to re-establish their ability to coerce fellow prisoners and prison staff.

Gang leaders blamed

Jeffrey Beard, head of the California Department of Corrections and Rehabilitation (CDCR), said most of the prisoners participating in the hunger strike are doing so under extreme pressure from gang leaders, who are alleged to intimidate prisoners that don’t obey orders.

Solitary units, Beard added, are necessary to keep gang leaders from ordering killings and advancing their criminal activity while incarcerated.

Prison officials say not all SHU inmates are in solitary confinement.

- Those in power in California have a very simple, humane task in front of them. That is, sit down and have meaningful negotiations with the strikers, that would do substantial work in avoiding continued suffering - Isaac Ontiveros, Prisoner Hunger Strike Solidarity spokesman

Some units have access to television and law libraries, and are allowed visitors every weekend. Many inmates have cell mates, can earn degrees, and are allowed to send and receive letters.

In a Los Angeles Times op-ed, Beard said his department created the units in the early 1970s in response to rising prison gang violence and the deaths of 11 corrections employees at the hands of inmates.

Solitary units, he added, have saved lives inside and outside prison walls.

"We are very concerned about the inmates engaging in the strike," said Terry Thornton, a CDCR spokesperson.

"The hunger strike is impeding progress on reforms we implemented last fall, which include a case-by-case review of every validated [gang-affiliated] inmate."

As the hunger strike moves into the fifth week, 200 inmates have been fasting continuously since July 8.

Some have lost as much as 15 percent of their body weight, according to reports, while dozens of strikers refuse to be weighed.

One striker, Mutope Duguma, who wrote to the San Francisco Bay View, says he has lost 33 pounds. "I know things will start to turn for the worse real soon," he said.

"There's general concern for their health, like there would be under the best circumstances if somebody hasn't eaten for a month,” said Isaac Ontiveros, a spokesperson for the Oakland, California-based Prisoner Hunger Strike Solidarity.

"Those in power in California have a very simple, humane task in front of them. That is, sit down and have meaningful negotiations with the strikers, that would do substantial work in avoiding continued suffering."