The brother of murdered Meredith Kercher has called for the US to agree to Amanda Knox being extradited immediately back to Italy to serve her prison sentence for the killing.

Lyle Kercher, who was in court when judges handed down the sentence after a retrial, said he saw no reason for the American authorities to delay her return.

Knox has been in Seattle since she was cleared on appeal of the killing and freed from prison in October 2011.

Yesterday, Lyle and other family members spoke at a press conference in the Italian city of Florence, where the retrial judgment was given on Thursday.

He said: "If somebody is found guilty and convicted of a murder, and if an extradition law exists between those two countries, then I don't see why they wouldn't. I imagine it would set a difficult precedent if a country such as the US didn't choose to go along with laws that they themselves uphold when extraditing convicted criminals from other countries."

After Knox revealed in the US that her lawyer was to pass on a letter to the Kercher family in which she explained how 'incredibly difficult' the long-running case must be for them, Ms Kercher's sister Stephanie said her family did not want to read it.

She also revealed that they did not want to meet Knox, telling reporters: "It's not something that we would want to do at the moment and I can't say that we ever will."

She said the family was still on a "journey for the truth" and admitted they were adjusting to the possibility of never knowing what had happened to Meredith.

"You can't ever really get to a point where you just start to remember Meredith, solely because it is following the case, coming over to Italy and everything associated with it.

"But the verdict has been upheld this time so we hope that... obviously, come the end of the trial, we are nearer the truth and an end so that we can just start to remember Meredith for who she was and draw a line under it, as it were."

The Kercher family said the six years of legal wrangling since Meredith was killed, which has done little to clear up the mysteries surrounding the case, had compounded their loss.

Sister Stephanie said she had not been able to properly grieve due to a drawn-out struggle to establish the basic facts of the night their sister was killed.

"It may be that we never know the truth about what happened that night," she said.

On Thursday, following the verdict, Meredith's journalist father John Kercher said: "No matter what the verdict, it was never going to be a case of celebrating anything."