French police have arrested a 55-year old German national after his wife was discovered naked and with broken bones following 12 years in captivity at the couple's home.

A police source said the woman, also German, was found with her head shaved in a bedroom of the apartment. She had multiple injuries, including broken bones, and was undernourished.

She had probably been tortured, the source said.

The state of her health was "not good", local prosecutor Olivier Glady told AFP after her examination by the local forensic medicine service. 

Police said the bedroom and other rooms in the apartment were closed off with metal wire at the flat in Forbach in eastern France, near the German border.

There were around 10 cats in the apartment, Glady said.

A police source said the woman, who is 53, got access to a phone and called police in Wiesbaden, western Germany, who in turn alerted their French colleagues.

She was rushed to hospital with fractures in both legs, and her fingers.

Over the years, the man had told everyone in the building that his wife had cancer, said Alicia, a neighbour.

"I never saw the lady, I don't think she ever left the house," she said.

"Sometimes I heard screams," Alicia said, but added that she thought they were caused by the illness.

About the man, she said: "Very polite, nice".

Erika, another witness who lives in an adjacent street, said she last saw the woman "maybe 10 years ago", and thought that "maybe she had died, or moved out".

Police are building a case against the man for kidnapping, aggravated rape, and acts of torture and barbarism, French broadcaster BFMTV said.

Broadcaster RMC said police found a diary-type notebook in which it was believed the man kept a record of his actions and of the times he gave his wife food but the prosecutor did not confirm this.

The man is jobless and believed to have worked in German industry previously, Glady said.