Sri Lanka's government is one of the world's worst perpetrators of enforced disappearances, US-based pressure group Human Rights Watch (HRW) says.
The security forces are accused of abducting hundreds of people
An HRW report accuses security forces and pro-government militias of abducting and "disappearing" hundreds of people - mostly Tamils - since 2006.
Sri Lanka's government says HRW has exaggerated the scale of the problem.
In a separate development, a team of foreign judicial experts has announced its withdrawal from the country.
HRW says many of the missing are young Tamil men targeted on suspicion of links to Tamil Tiger rebels.
But their conclusions have been flatly rejected by the Sri Lankan government.
Concern has been expressed over the disappearance of aid workers
Foreign Secretary Palitha Kohona says there has been a "steady decline in disappearances over the last 12 months" because of new measures taken by the government.
"Unfortunately Human Rights Watch has tended to exaggerate the real situation," Mr Kohona said.
He said the group's "unfair" report was based on unsubstantiated claims and "anecdotal evidence", while the government's own investigations into disappearances were proceeding quickly.
Tens of thousands have died since separatist Tamil Tiger rebels began fighting the Sri Lankan government more than three decades ago.
HRW said several hundred cases of disappearances had been reported since 2006, when fighting between the Tamil Tiger rebels and the government intensified.
The rights group said the majority of cases "indicate the involvement of government security forces - army, navy or police".
The government insists that it does respect human rights
The group said pro-government armed Tamil groups - such as the faction led by renegade rebel commander Col Karuna - had also been implicated in the abductions and disappearances.
While most of the victims were members of Sri Lanka's ethnic Tamil minority, HRW said, some Muslims and Sinhalese had also been targeted.
These included journalists, aid workers, clergy and teachers, HRW said.
Many of the missing were feared dead.
HRW said the Sri Lankan government's response to the disappearances had been "grossly inadequate".
It urged the government to reveal the whereabouts of the missing and prosecute those responsible.
The rights group said the number of disappearances carried out by the Tamil Tigers in government-controlled areas was relatively low.
But it said, the Tigers were responsible for targeted killings, forced child recruitment, bomb attacks on civilians and the repression of basic rights in areas they controlled.
'Lack of respect'
Meanwhile, a team of foreign judicial and forensic experts says it is pulling out of the war-torn island because the government has failed to investigate a series of high-profile cases - including the killing of aid workers.
The International Independent Group of Eminent Persons (IIGEP) said a government probe into abuses did not meet even basic minimum standards.
The team comprises experts from Australia, Britain, Canada, India, Japan, France, The Netherlands, the US, the European Union and the UN High Commissioner for Human Rights.
The IIGEP said its suggestions had been ignored or rejected by the government, whose correspondence with them was "characterised by a lack of respect and civility".
"There has been and continues to be a lack of political and institutional will to investigate and inquire into the cases before the (government) commission," an IIGEP statement said.
Correspondents say that the group's conclusions are a major blow to the Sri Lankan government.