copyright 2009, The Washington Post NEW YORK -- The State Department's top war crimes official called on Sri Lanka on Thursday to conduct a 'genuine' investigation into alleged war crimes by Sri Lankan troops and Tamil rebels during the bloody final months of the country's 25-year-long civil war. The appeal by Stephen Rapp, the U.S. ambassador at large for war crimes issues, came hours after his office presented Congress with a detailed account of alleged atrocities during the conflict that suggests both sides may have violated international law and committed crimes against humanity. The 68-page document, which relies on internal reports from the U.S. Embassy in Colombo, satellite imagery and accounts from international relief agencies and news organizations, paints a grim portrait of the conditions endured by hundreds of thousands of Tamil civilians caught between two ruthless adversaries. Between 7,000 and 20,000 civilians were allegedly killed in the country's northeast from January to May, when the Tamil Tigers were defeated, according to U.N. and independent estimates. 'We are certainly calling on the government, as part of the reconciliation process, to develop an accountability process that respects the interests of all,' Rapp said in an interview. Noting that Sri Lankan authorities have insisted they can conduct a credible internal investigation into alleged abuses, he said, 'We are going to take them at their word and follow that process.' Sen. Patrick Leahy, D-Vt., chairman of the Appropriations subcommittee on the State Department and foreign operations, who authorized the report, said that it 'eliminates any reasonable doubt that serious violations of the laws of war were committed by both' sides. 'A full and independent investigation is needed, and those responsible must be held accountable,' he said. Rapp also expressed concern for the plight of more than 250,000 displaced ethnic Tamils who remain confined to government-controlled camps. He said the imminent seasonal monsoon could inundate the camps and cited reports from U.S. and other sources that Sri Lankan authorities had abducted young males at government-run screening centers for the displaced. In response to the report, Sri Lanka's Foreign Ministry released a statement Thursday saying that the allegations contained in it 'appear to be unsubstantiated and devoid of corroborative evidence.' It said that Sri Lankan security forces had been 'engaged in a humanitarian mission' aimed at freeing captive civilians from the 'clutches' of the separatist group and that they had been 'scrupulous in affording protection to the civilians and safeguarding their welfare.' The U.S. report documents nearly 170 incidents between Jan. 2 and May 18 in which thousands of Tamil civilians were allegedly gunned down by Tamil rebels seeking to use them as human shields or killed by indiscriminate government shelling. It portrays a population enduring violence, starvation and severe shortages of drugs, including anesthesia for amputations. The Obama administration has faced criticism from human rights groups and political analysts for failing to confront Sri Lanka more forcefully over reports of indiscriminate shelling of displaced civilians as it moved in to crush the Liberation Tigers of Tamil Eelam. Rapp defended the U.S. response, saying then-Ambassador to Sri Lanka Robert Blake had pressed for a humanitarian pause in the fighting and routinely raised concerns about Sri Lanka's conduct during the war. The report includes multiple allegations that the Tamil Tigers forcibly recruited children as young as 11 to fight, killing relatives who objected. It also alleges that the rebels used suicide bombers to kill government officials and police and that they regularly fired on civilians attempting to flee the war zone. It cites a Feb. 13 report to the U.S. Embassy by a foreign government alleging that rebels killed 60 civilians who were trying to escape by boat. Also detailed in the report are numerous alleged instances of government shelling of civilian populations, hospitals, churches and schools in rebel-controlled territory, often in areas designated by the government as no-fire zones. The U.S. Embassy reported several cases in which large numbers of civilians were reportedly killed in shelling, including nine in the April 29 bombardment of the Mullivaikkal Hospital. 