Al Jazeera has obtained photographs that appear to show Sri Lankan army atrocities against Tamil civilians and captured or surrendered Tamil fighters in the final days of the Sri Lankan civil war in early 2009.
Human Rights Watch and Amnesty International say that the new photographic evidence warrants an independent, international, investigation.
Sri Lanka rejects outright any probe of its military's conduct.
Along with the International Crisis Group, the two groups Wednesday again rejected the Sri Lankan government's own inquiry as "warcrimes whitewashed".
Earlier this year, the ICG published the findings of a detailed investigation of war crimes in the final months of Sri Lanka's war.
The evidence gathered by ICG "provides reasonable grounds to believe the Sri Lankan security forces committed war crimes with top government and military leaders potentially responsible," the report's executive summary said.
Sri Lanka has responded by denouncing the groups' criticism as "colonial, patronising and condescending".