A top foreign panel invited by Sri Lanka to oversee investigation into 16 cases of serious rights abuses says it is resigning because there is lack of political and institutional will to probe the cases.
The 11-member International Independent Group of Eminent Persons was created by the government, two years ago.
The panel says the government's investigation has fallen short of transparency and compliance with basic international norms.
The cases being observed by the panel include the massacre of 17 local aid workers of a French charity in 2006.
The panel quit as a new report by the American-based Human Rights Watch said hundreds of people "disappeared" or were abducted in Sri Lanka in 2006 and 2007, when hostilities with the Tamil Tiger rebels intensified.
The group says the involvement of government security forces - army, navy or police - is indicated in a majority of these cases.
The government says many people listed as missing went away without informing their family members.
Anna Neistat researched the Human Rights Watch report. She disputes that position.
"In many cases, we have proof that, before people disappeared, before they went missing they were last seen in custody of army or navy officers, or they were arrested by the police…. and since then all traces of them have been lost," she said.
The report says many of the victims are ethnic Tamils and were targeted because of their alleged links to the rebels. It says others who have disappeared include journalists and aid workers. It says many of the victims are feared dead.
It has called for a United Nations mission to monitor the rights abuses.
The government has repeatedly denied security forces are involved in rights abuses and rejected previous calls for international monitoring.
Some analysts say the government's credibility will suffer a serious blow with the departure of the international panel.
Rights workers say the situation deteriorated after security forces and rebels resumed fighting, following a four-year lull in the long, drawn out civil war.