A US-based human rights group has accused the Sri Lankan government of what it calls a shocking rise in abuses by its security forces.
Fighting between the army and rebels has recently intensified
Human Rights Watch said there had been an increase in unlawful killings, enforced disappearances and other abuses over the past 18 months.
It said the rights situation had become worse as the island slid back into war.
The government admitted there were cases of disappearances but said it was taking every step to investigate.
Human Rights Watch said there had been an increase in abuses since conflict resumed between the Tamil Tiger rebels and the government, effectively ending a 2002 ceasefire.
The group's Asia director, Brad Adams, said the government had apparently given the green light to its security forces to use the tactics of dirty war.
The report said the Tamil Tiger rebels stood accused of targeting civilians, extortion and the use of child soldiers.
But Mr Adams said that was no excuse for what he described as the government's campaign of killings, disappearances and the forced returns of the displaced.
According to Human Rights Watch, from January 2006 until June this year, more than 1,000 abductions had been reported in Sri Lanka.
A Sri Lankan government spokesman for security and defence, Keheliya Rambukwella, told the BBC that every disappearance was being investigated.
"The government is taking every possible step... The disappearances have been there, it is true. All of the cases that are reported are investigated."
Mr Rambukwella denied the security forces were involved in an increase in abuses.
"These allegations are not based on credible evidence," he told the AFP news agency.
The BBC's Roland Buerk in Colombo says the report comes as humanitarian agencies mark one year since the bodies of 17 workers for the French charity Action Against Hunger were found in the east of the island.
They were shot dead as the town of Muttur changed hands between the rebels and the government.
International ceasefire monitors blamed the military. The government denied it, but no-one has been brought to justice.