Ex-war crimes prosecutor Carla Del Ponte made the claims in a book
Human Rights Watch has called for an investigation into claims that ethnic Albanians in Kosovo abducted and killed Serbs, and may have sold their organs.
The claims involve about 400 Serbs who went missing after the war in 1999.
The rights group says new information has come to light, some of it in a book by former UN war crimes prosecutor Carla Del Ponte.
But Albanian PM Sali Berisha said the claims were like "an Agatha Christie novel" and had been investigated.
Kosovo's assembly is due to convene in the coming weeks to discuss whether to sue Ms Del Ponte for allegedly tarnishing the image of Kosovo.
Ms Del Ponte said in her book that she had learned from "credible journalists" that organs were being sold.
She said the sources had told her that between 100 and 300 mostly Serb civilians were taken from Kosovo into Albania, where "doctors extracted the captives' internal organs".
At the time UN and Nato forces were being deployed to Kosovo as the war between Serbian forces and separatists was ending.
Human Rights Watch (HRW) said it had reviewed investigations carried out at the time, and that it believed the allegations that Serbs were abducted were "serious and credible".
Fred Abrahams, senior emergencies researcher with HRW, called on Kosovo and Albania to "show their commitment to justice and the rule of law by conducting proper investigations".
He added that the particular allegations of organ trafficking were "suggestive but far from complete".
But Mr Berisha said: "There has been a clear investigation by both national and international prosecutors and they have found no evidence."
He added: "Carla Del Ponte never presented an official request to my government on the matter."