A father facing charges of sexual abuse in a Dutch court had held six of his nine children captive for nearly a decade at an isolated farmhouse, telling them "bad spirits" would enter their bodies if they talked to outsiders, prosecutors said on Tuesday.

Gerrit-Jan van Dorsten, 67, is charged with unlawful detention, child abuse and money laundering.

Now ailing after a stroke, he cannot speak and did not attend the pretrial hearing in the city of Assen during which his custody was extended by three months.

In October, police found van Dorsten and five adult siblings at the farm in Ruinerwold in the northern Netherlands after a sixth sibling escaped and reached a nearby village bar. Local people alerted police.

"All the children report physical punishment if they were deemed to be under the influence of spirits. This happened from a very young age, with children as young as four or five years old," prosecutor Diana Roggen told the judges.

"The punishments would consist of beatings, sometimes with a stick or other objects, pulling of their hair, sometimes making them sit in a cold bath for hours. Sometimes, they were choked to the point where they became unconscious."

Judges granted a prosecution request that van Dorsten undergo neurological and psychiatric evaluations and said defence lawyers may question the children, four males and five females who are now all adults.

The six youngest children were kept in seclusion since birth and separated from their elder siblings. A 15-year-old daughter was moved to another town, while a 12-year-old son was forced to live alone in a caravan.

Van Dorsten told the children the death of their mother in 2004 was their fault for having contact with the outside world.

"I had a bad spirit and I did not want to transfer it to them," one of the children told prosecutors during earlier questioning.

The children were watching proceedings remotely, presiding Judge Herman Fransen said. They may testify at a later stage of the trial.

The allegations by the children were confirmed by diaries written by van Dorsten seized during a search of the farmhouse, the prosecutor said.

Between 2007 and their discovery last year, van Dorsten withheld food, drink or medical treatment. Some of the older children have said their father forced some of them to perform sex acts on him between 2004 and 2008, prosecutors said.

Roggen said van Dorsten also told the children "a female spirit, the spirit of their mother or another spiritual wife" had entered their bodies to justify sexual acts with them.

The six siblings and their father had lived on the farm since 2010, and had never had their births registered or been to school, as required by Dutch law.

Three more older siblings had left the family before they went into seclusion.

Two of them have now come forward with accusations of sexual abuse when they were young teenagers.

Van Dorsten is in a prison hospital where police have been unable to question him because an untreated stroke in 2014 rendered him unable to speak.

A second suspect, Austrian Josef Brunner, 58, a follower or accomplice of van Dorsten who paid the rent on the farmhouse, is charged with endangering the health of others and unlawful detention.

"In my conscience, [I know] I did not rob anyone of their freedom," Brunner told judges on Tuesday. "This feels like a witch-hunt."

Brunner's lawyer, Yehudi Moszkowicz, countered charges of keeping the children against their will, citing interviews in with they said could have left the property, but chose not to.

The children, all now over 18, have not spoken publicly but have given statements through a Dutch filmmaker.

The four eldest said in November that they support the criminal case against their father. Separately, the five youngest children - those found at the farm - say they do not support the charges.

The five younger children are in counselling and are doing well under the circumstances, prosecutors said.