An elderly Austrian man has confessed to holding his daughter captive in a basement cell for 24 years and fathering seven children with her, police said Monday.

The 73-year-old man allegedly held his daughter Elisabeth, now 42, and three of their children in a soundproof chamber below an apartment block in Amstetten, a town near the Austrian Alps.

Police say he raised three of the children with his wife, who knew nothing of the secret cellar chamber.

The man told police he burned the body of the seventh child, who allegedly died of neglect shortly after birth, in an incinerator.

Elisabeth was originally reported missing in 1984 when she was 18 years old. She was located by Amstetten police on Saturday evening after they received a tip. She is now in psychiatric care with her children, two of whom are said to be in poor health.

"They are in a life-threatening condition," the head of the clinic where they are being treated told reporters. "All of them are being given intensive medical treatment. I can give no prognosis. Only God knows that."

On Monday, police released photos of the underground cell which included a small bathroom and a tiny bedroom. The cell was made of reinforced concrete; Elisabeth was prevented from leaving by an electronic keyless locking system.

The suspect, known as Josef F., was being held in a pretrial detention centre. He faces up to 15 years in prison, if charged and convicted of rape, the most serious of the allegations under Austrian law.

Hans-Heinz Lenze, a senior local official, said the suspect's wife apparently had "no idea'' of what went on and was devastated.

"You have to imagine that this woman's world fell apart,'' he told the Associated Press.

Austrian broadcaster Steven Crilley said neighbours in the sleepy, working-class town were reeling, as many had interacted with members of the family.

"The ones above ground led apparently very normal lives," he told Â鶹´«Ã½net on Monday. "People knew the family. They weren't so social but that wasn't a big problem."

Austrians are reeling from the shock of the tragic story, particularly since a similar one surfaced just two years ago when a young woman was found to have been held in a basement cell for eight years after being kidnapped.

Natascha Kampusch, who was 10 years old when she was kidnapped in Vienna in 1998, issued a statement Monday offering emotional and financial help to Elisabeth and her family.

With files from the Associated Press