Image caption A lawsuit has been filed on behalf of prisoners in solitary confirement for more than 10 years

A hunger strike by California prisoners has entered its fourth day, with 29,000 inmates protesting against lengthy solitary confinement sentences at the state's high-security prisons.

California's prisons chief says the protest is detrimental to their cause.

Many of the roughly 3,800 prisoners in solitary confinement have been deemed to have gang ties, and some have spent more than a decade in isolation.

It is the third such hunger strike in California prisons in two years.

But the strike that began on Monday when 30,000 inmates refused meals is the largest in the state's history. The number of striking prisoners fell below 29,000 on Wednesday.

'Make their point'

"I don't think it helps anything to do this," Corrections Secretary Jeffrey Beard said on Thursday in his first comments on the strike.

"Much of what they're asking for is being done. It's just not being done fast enough for them... the hunger strike actually interferes with the process."

After the two previous strikes, the corrections department began a programme to reduce isolation sentences. Hundreds have either already been released from solitary confinement or have been marked for release.

But the programme was suspended as the strike began on Monday. Mr Beard told an oversight hearing that corrections authorities would not make concessions to the prisoners.

"I think the department has pretty much done what it can do," Mr Beard told the Associated Press on Thursday. "My hope is that they sort of make their point, get the thing over and we can go back and start doing the reviews."

The strikers' supporters say California's programme does too little to avoid or limit the confinement sentences.

22-24 hours a day

"They're asking for a more humane set of conditions that aren't designed to destroy people," said Claude Marks, a spokesman for the Prisoner Hunger Strike Solidarity Coalition.

"If that's [Beard's] position, that [California] has nothing else to offer, then that explains why there's an issue."

In one high-security prison near the Oregon border, Pelican Bay, more than 1,000 prisoners remain in solitary confinement.

About 500 have been in isolation between 22-24 hours a day for more than a decade, with dozens spending more than 20 years in the cells, according to a lawsuit filed on their behalf.

Three other high-security prisons in California have similar programmes.

According to the Los Angeles Times, those formerly in isolation have been released into the general prison population either because they no longer qualified for isolation, were part of a four-year "step-down" programme, or had agreed to inform on other prisoners.