New York-based Human Rights Watch said Yogi, who was the political chief of the LTTE, and Tamil Tiger fighters, who surrendered to the army were last seen being trooped into a bus in the war zone in May 2009 and remained untraced till date.
Most of them are known to have been detained in the Vadduvaakal area, just south of the strip of land in northeastern Sri Lanka where the final battle between the LTTE and government forces occurred. At the time, the area was controlled by the Sri Lankan army's 59 Division.
Demanding that a UN panel investigating the wartime actions by the Sri Lankan Army and the Tigers probe their disappearance, the rights group said it had several video clips showing a senior Tiger commander, known as "Col Ramesh" in military custody, though the government had declared him killed in fighting.
Quoting witnesses, the rights group said said the wife and two children of a prominent rebel leader were also seen being taken in the same bus, but there was no information what happened to them.
Human Rights Watch said unresolved enforced disappearances should be part of the mandate of a proposed United Nations investigation into laws-of-war violations by both government forces and the LTTE.
The Sri Lankan government should account for everyone who was taken into custody at the end of Sri Lanka's 26-year-long armed conflict Despite numerous requests from families for information about their relatives, the authorities do not appear to have conducted any serious investigations, it said.
"The Sri Lankan government needs to respond to all allegations of disappearances with more than a ritual blanket denial," said Brad Adams, Asia director at Human Rights Watch.
"Family members of the disappeared have the right to know if their loved ones are alive or dead."