An Italian judge presiding over the retrial of American student Amanda Knox has ordered new DNA tests on the knife that prosecutors say was used to kill her British roommate in 2007.

On the trial's opening day on Monday, presiding Judge Alessandro Nencini said the court agreed to test one DNA trace not previously examined on the knife that prosecutors allege killed British student Meredith Kercher; the trace had previously been deemed too small to test.

Knox and her Italian ex-boyfriend Raffaele Sollecito were found guilty in 2009 of murdering 21-year-old Meredith Kercher.

They were acquitted on appeal in 2011 but the acquittal was later quashed by Italy's supreme court.

Neither appeared in court on Monday for the first hearing in the retrial. Knox, now back home in Seattle, has said she will not be returning to Italy.

Nencini will also hear new testimony from jailed Naples mafia member Luciano Aviello, who previously said his brother killed Kercher. He is due to appear in court on Friday.

The new checks on the presumed murder weapon - a kitchen knife found in Sollecito's house - will examine a trace that was not previously tested because experts said it was too small to produce reliable results.

Nail-bitten fingers

The court will also assess photographs of Sollecito's nail-bitten fingers which the defence have presented.

The supreme court overturned the acquittal of Knox and Sollecito in March, citing "contradictions and inconsistencies" and paving the way for the retrial.

Kercher was found with more than 40 wounds, including a deep gash in the throat, in the apartment she shared with Knox in Perugia, a picturesque town in the central Umbria region that attracts students from around the world.

Knox, 26, has denied involvement in the killing. She told US television this month that "common sense" told her not to return to Italy. She is not obliged to attend the hearing and can be represented by her lawyers, who said she is watching the retrial closely from home in Seattle.

Sollecito, 29, who has also protested his innocence, plans to attend some of the hearings, his father Francesco said, adding he was confident his son's innocence would be confirmed.

"Deeper examination can only demonstrate what we already know, that Raffaele Sollecito has nothing to do with what that poor girl had to suffer," he told reporters.

Source: Agencies