Amanda Knox vowed on Friday to fight her second conviction for the murder of British student Meredith Kercher in 2007 while the two were students together in the Italian university town of Perugia.

Speaking on U.S. television a day after her conviction by a court in Florence, the 26-year-old American said she would never willingly return to Italy to serve the 28-1/2-year sentence handed down by judges.

"I'm going to fight this until the very end. And it's not right, and it's not fair and I'm going to do everything that I can," she told ABC News' Good Morning America program on Friday.

The family of the victim urged the United States to agree to extradite Knox if her conviction is upheld after a final appeal process expected to conclude in 2015.

U.S. State Department spokeswoman Marie Harf told reporters on Friday that officials will continue to monitor the case.

Knox and her former Italian boyfriend, Raffaele Sollecito, were both found guilty of killing the 21-year-old Kercher, who was found stabbed to death in an apartment the two young women shared in Perugia.

Sollecito on Friday also vowed to contest the conviction.

Italy Knox Trial

Amanda Knox's ex-boyfriend, Raffaele Sollecito, is not allowed to leave Italy after being convicted in Meredith Kercher's death. (Antonio Calanni/The Associated Press)

"To me, all of this makes no sense, so I will fight until the end," he told NBC News in an interview after being detained, then released, by police following the verdict.

Knox has remained in her U.S. hometown of Seattle since being released from prison in 2011 after an appeal overturned an original conviction and freed her and Sollecito after four years in custody.

Extradition fight likely

Neither her sentence nor the 25-year prison term handed to Sollecito will have to be served pending further appeals, and a prolonged legal fight is now likely.

Sollecito left the court hours before the verdict was delivered and was found by police in the early hours of Friday between the northern towns of Udine and Tarvisio, less than 10 kilometres from Italy's border with Austria.

He told NBC he was so confident that he and Knox would be found not guilty that he had planned a trip outside of Italy, according to the television network.

As soon as he learned of the verdict, he returned to Italy to surrender his passport, but officials detained him before he could get to a police station, he said, according to NBC. He was released after several hours.

His lawyer, Luca Maori, denied he was trying to escape the country, having left Thursday's hearing early because of stress.

Under the terms of Sollecito's sentence, authorities took his passport and have instructed him not to leave Italy. For the moment he is free to travel around inside the country.