SAN FRANCISCO (REUTERS) - California prison authorities warned thousands of hunger-striking inmates on Thursday that they could face discipline for illegal "mass disturbances," and confirmed that more than 12,000 prisoners missed nine consecutive meals in the past three days.

The hunger strikers, who are also refusing work assignments, are protesting what prisoner advocates describe as the state's inhumane solitary confinement practices, which can include locking inmates in isolated cells for up to 23 hours a day.

The action launched on Monday by inmates at over two dozen correctional facilities marked the largest prison hunger strike in California history, according to the Los Angeles Times. It is nearly twice the size of a 2011 strike that at its peak involved 6,500 inmates.

"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,"state corrections officials said in a statement in response to the strike.

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.

The department said the strike was led by prison gangs and that "mass hunger strikes, work stoppages and other disruptions"could potentially affect safety and security behind bars.

The strike comes at an already challenging time for the prison system in the most populous US state, which has been ordered by a federal court to reduce prison size by 10,000 inmates this year to ease crowding.

The state has begun housing many low-level prisoners in county jails. Governor Jerry Brown has been feuding with federal judges over demands the state continue to reduce inmate numbers.