Danish inventor Peter Madsen has admitted to dismembering Swedish journalist Kim Wall on board his submarine in August and dumping her body parts in the sea, but still denies killing her, police say.

Key points: Danish police say Mr Madsen will also be charged with sexual assault

Danish police say Mr Madsen will also be charged with sexual assault Wall had 14 interior and exterior stab wounds to the genitals, police say

Wall had 14 interior and exterior stab wounds to the genitals, police say Mr Madsen denies the murder and sexual assault charges

Mr Madsen said Wall died from carbon monoxide poisoning inside the submarine, while he was on the deck of the vessel, Danish police said in a statement.

He had previously claimed she had died when she hit her head on a hatch, a theory that was disproved when divers recovered her severed head which showed no signs of fractures.

Danish police said they would now extend the charges against Madsen — which already include murder and mutilating Wall's body — to include sexual assault without intercourse, based on 14 interior and exterior stab wounds to the journalist's genitals.

Sorry, this video has expired Danish police search the Nautilus as they hunt for Kim Wall's body

Mr Madsen denies the murder and sexual assault charges.

Wall, a freelance journalist who was researching a story on Madsen, went missing after he took her out to sea in the 17-metre submarine UC3 Nautilus in August.

On August 23, police identified a headless female torso that washed ashore in Copenhagen as Wall's.

Swedish journalist Wall died while researching a story on Madsen. ( AP: Tom Wall )

Earlier this month police said they had also recovered Wall's head and legs. They said they found no skull fracture, despite Mr Madsen's earlier account that she had been hit in the head by the hatch cover.

The submarine is one of three that Mr Madsen built, and one of the largest privately built ones in the world. It could carry eight people and weighed 40 tonnes fully equipped.

Mr Madsen has voluntarily accepted the prosecutor's request to extend his custody until November 15, his lawyer said, meaning he will not appear in court on Tuesday (local time) as initially planned.

The UC3 Nautilus was Peter Madsen's third submarine design. ( Wikipedia Commons: Frumperino )

Reuters/ABC