An Italian court on Thursday pronounced US student Amanda Knox guilty of murder and sentenced her in absentia to 28 years and six months in prison in what she said was an "unjust verdict".

Chief judge Alessandro Nencini read out the ruling to a stunned silence in the Florence courtroom and Knox's lawyers said they would appeal in the high-profile and long-running case.

"I am frightened and saddened by this unjust verdict. Having been found innocent before, I expected better from the Italian justice system," Knox said in a statement from her home in Seattle.

"This has gotten out of hand," said the 26-year-old, who is completing a degree in creative writing at the University of Washington after returing to the United States following a previous acquittal.

After 12 hours of deliberations, the court's two judges and eight jurors also reinstated a murder conviction for Knox's former Italian boyfriend Raffaele Sollecito and sentenced him to 25 years.

Dressed in a blue sailor jacket and wearing sepia-coloured aviator sunglasses, Sollecito had attended a court hearing earlier on Thursday.

But he was not present when the verdict was read out and will have his passport withdrawn to stop him fleeing before his final appeal is heard.

British student Meredith Kercher was found with her throat slit on November 2, 2007 in the cottage she shared with Knox in the mediaeval university town of Perugia where she was on an exchange programme.

Kercher's brother Lyle and sister Stephanie sat impassively in the court as the verdict was read out.

Lyle Kercher said afterwards that it was "not a moment for celebration" but added: "We've trusted what the prosecution has been doing all along".

"It's hopefully a step towards reaching a conclusion," Kercher said before the ruling.

"To lose someone you love so dearly is hard enough, especially in the way she died, but that has been compounded by the fact that it has gone on for six years and three months," he added.

The Kercher family lawyer, Francesco Maresca, expressed "great satisfaction" with the decision.

Knox and Sollecito were first convicted of the murder in 2009, then acquitted in 2011 on appeal.

The supreme court last year then ordered a re-trial, leading to the guilty verdicts issued on Thursday.

Sollecito's lawyer Giulia Bongiorno said she was willing to continue a "ping pong" of appeals until her client was finally absolved of the crime.

"This conviction was based on nothing. It's absurd. Raffaele Sollecito is innocent," another Sollecito lawyer, Luca Maori, told reporters.

'A real blow'

Luciano Ghirga, a lawyer for Knox, said the verdict was "a real blow" and also vowed to press on.

An extradition procedure for Knox can only be launched following a definitive ruling from the supreme court, which could take months or years.

Pending that verdict, Knox will not have restrictions on her travel as the judge on Thursday did not issue any detention order against her.

Legal experts say an extradition from the United States is possible but extremely unlikely.

Prosecutors had said during the trial that the murder may have been the result of a sex game turned violent due to tensions between Kercher and Knox.

The defence had dismissed this and said there was a lack of conclusive DNA evidence putting Knox and Sollecito in the bedroom where the murder happened.

A third accused, local drug dealer Rudy Guede, is currently the only person in prison for the murder.

But investigators say that multiple stab wounds on the body -- apparently from two different knives -- indicate he could not have acted alone.

Knox and Sollecito have always protested their innocence and Guede has changed his story several times but maintains he did not commit the murder although he was in the house when it happened.

Prosecutors had asked for Knox to be given 30 years and Sollecito 26 years, saying the punishment should be harsher for her because she initially accused someone else of the crime, bar owner Patrick Lumumba.

Knox has said the accusation of Lumumba, as well as her since retracted memory of hearing Kercher's screams, were due to aggressive police interrogation.

Knox now says she was at Sollecito's house all night.

She explained her absence in an email to the court during the trial saying: "I am not a monster".