A 15-year-old girl who murdered her mother after becoming obsessed with ISIS videos has been jailed.

Danish teenager Lisa Borch and her Islamist boyfriend, 29, had been watching videos of British hostages David Haines and Alan Henning being beheaded before they fatally stabbed her mother at least 20 times, according to reports.

Borch killed her mother last October with a kitchen knife and then rang the police to report the murder.

She told them "I heard my mother scream and I looked out the window and saw a white man running away. Please come here, there is blood everywhere."

Borch sat disinterested on her phone when the police arrived and simply pointed up the stairs when asked where her mother was.

Tine Römer Holtegaard, an artist who lived with her twin daughters and husband, was found by police covered in blood in her bedroom.

An examination of her computer found Borch had been watching videos of beheadings on repeat "the whole evening long."

Her 29-year-old boyfriend, Bakhtiar Mohammed Abdullah, was not at the scene when police arrived, but his fingerprints were found in the victim's bedroom.

Borch had become obsessed with militant Islam after a relationship with an unnamed Muslim man, who later left her to go back to Sweden to be with his wife and children.

Later, she met Iraqi-born Abdullah at a refugee centre near her home and it is believed the two had planned to flee to Syria to fight for ISIS together.

Borch's twin sister had left the family home previously due to endless rowing between the mother and daughter, usually centred around Borch and Abdullah's relationship.

The prosecution told the court: "This murder was cold-blooded, ice cold and committed in a bestial manner."

Neither Borch nor Abdullah admitted to being the one that wielded the knife and both were convicted of murder.

Borch has been sentenced to nine years in prison, one of which will be in a young offender's institution.

Abdullah was sentenced to 13 years in prison, after which he will be expelled from Denmark.