A court in France sentenced a man to 18 years in prison on Friday for stabbing and then burning alive his 15-year-old girlfriend.

The 2019 killing of the secondary school pupil, identified in court only as Shaina, revived outrage in France over the number of women who die at the hands of intimate partners.

The court was told that the accused, a 17-year-old high school student at the time, had lured Shaina to a shed in the town of Creil north of Paris to kill her and burn her body.

Post-mortem forensic examination revealed "multiple wounds" inflicted by a knife, but also that Shaina was still breathing at the start of the fire.

According to public prosecutor Loic Abrial, the crime was "premeditated at every stage."

He had sought a tougher, 20 or 30-year sentence, but the court took the accused's status as a minor into consideration.

In response, Shaina's brother, Yasin, cried angrily.

"18 years! That's justice in France," he shouted at the special juvenile court in Oise, and had a tense exchange with the accused before collapsing and being taken to hospital.

Restrictions prevent the media from identifying the accused, who has staunchly maintained his innocence.

"Why, why?" he said in reaction to the verdict, following four-hour deliberations.

"You're wrong, I'm innocent."

Defence lawyer Elise Arfi said it was "too early" to comment on the possibility of an appeal.

The case made waves in France where a woman is murdered by her partner or former partner every three days, according to official figures.

Shaina's murder was particularly shocking, not only because of her young age, but also because two years earlier she had been the victim of sexual assault, for which four other young people were given suspended sentences ranging from six months to two years in prison.

Graphic images of her assault had been posted online with the aim, according to lawyers, of showing Shaina as a person "who they can sleep with but also get rid of".

Described by her mother as "funny and smiling", Shaina was probably in the early stages of pregnancy at the time of her murder, according to investigators.

The day before she was killed, she had gone out after a family dinner. In her handbag relatives had found a positive pregnancy test.

She attributed paternity to the accused, with whom she had had a relationship.

The prosecution maintained the accused had been "ready to destroy everything to save his image," and avoid the disapproval of his parents.

He could "be out in eight years" between pre-trial detention and sentence reductions, Shaina's family's lawyer, Negar Haeri, predicted.

"Justice doesn't care about violence against women," she said.