The rights group, in a news release Monday, also said it received information "suggestive but far from complete" that some abductees had internal organs extracted and transported out of Albania, where they were being held.
Human Rights Watch called on the Kosovar and Albanian governments to open independent investigations to determine what happened to hundreds of Serbs missing since the war. The release didn't suggest either government was involved in the abuses.
The allegations of abductions were made public in a book by Carla Del Ponte, formerly the chief prosecutor for the International Criminal Tribunal for the former Yugoslavia. Human Rights Watch said it had information independent of Del Ponte's book of post-war abuses in Kosovo.
The release said Kosovo Prime Minister Hashim Thaci and Albanian Prime Minister Sali Berisha have rejected the allegations as unsubstantiated and libelous. A Human Rights Watch letter to the leaders in early April asked for an investigation but the group said it hadn't received a reply a month later.