A couple facing trial over the torture and false imprisonment of their 13 children had four more charges filed against them on Friday in a California court.

The Riverside County district attorney filed three further charges of child abuse against David and Louise Turpin, and one count of felony assault by Mrs Turpin against one of her children.

The couple are pleading not guilty to all 42 charges, including torture, false imprisonment, abuse of a dependent adult and child abuse, as well as one count of lewd conduct with a minor against Mr Turpin.

Lawyers for the couple said they are both pleading not guilty to the additional charges filed on Friday.

"Today we filed an amended complaint in the Turpin case this afternoon," said John Hall, spokesman for the district attorney's office, speaking outside court after the hearing.