Human Rights Watch says there has been a "shocking" rise in killings, abductions and disappearances in Sri Lanka since a new round of fighting erupted last year between government forces and Tamil rebels.
A statement by the group says the government has apparently given its security forces a green light to use "dirty war tactics" in its fight against the rebels.
The report says 1,100 new cases of abductions and disappearances have been reported since January 2006.
One the authors of the report, Charu Hogg, says most of the victims are Tamil civilians, mostly men 18 to 50 years old. She says eyewitness accounts point to the involvement of government forces or armed groups acting with government complicity.
"If you look at the case of disappearances alone, most of these disappearances have occurred during curfew hours, when only the military or security forces were out on the streets. In other cases as well - abductions, extra-judicial executions, kidnappings or extortions - in a large number of these cases, government complicity has been pointed out," said Hogg.
The report says an armed faction that split from the rebels and now cooperates with the security forces is also guilty of abuses, including the recruitment of children.
The report also accuses security forces of blocking delivery of humanitarian aid to conflict areas, and forcing displaced people to return to areas that are unsafe.
The report says the Tamil rebels are also responsible for killings and extortion, but it says this does not forgive the government's alleged actions.
A Sri Lankan government spokesman has dismissed the accusations, saying they are not based on credible evidence. The government says it takes every possible step to monitor rights abuses.
Charu Hogg argues that the government has done little to address concerns about the increase in human-rights abuses.
"There is a building body of evidence, and yet no effort is taken to bring the perpetrators to justice. That seems to indicate the government does not seem to care at all what the security forces are doing - or the more insidious allegation would be, it [the government] is part of the conspiracy in which civilians are being targeted," added Hogg.
The report was issued a year after 17 local staff members of a Paris-based aid group, Action Against Hunger, were short dead in the northeast. Norwegian truce monitors blamed those killings on the security forces.
Fighting between the government and rebels intensified last year after the collapse of a truce. The rebels have been fighting since 1983 for an autonomous homeland for the minority Tamil community.