At least four Tamil political parties and several other civil society groups called for the return to the original hybrid judicial mechanism proposed in the UN Human Rights High Commissioner's report.
UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein had called for a hybrid special court, integrating international judges, prosecutors, lawyers and investigators to probe the alleged rights abuses during the civil war that ended in 2009.
A resolution to be moved by the US tomorrow in Geneva seeks to back a local mechanism with judges from the Commonwealth with foreign investigators -- in contrast to Al Hussein's report.
The Tamil groups said the resolution seeks to dilute the seriousness of the required investigation.
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"We note with deep regret that in the interest of arriving at a consensus resolution with Sri Lanka that many key components of the initial draft have been omitted".
Rights groups claim that the Sri Lankan military killed 40,000 civilians in the final months of the three decade-long brutal ethnic conflict with the separatists Liberation Tigers of Tamil Eelam (LTTE).
In a statement New York-based Human Rights Watch said: "This resolution makes it clear the time has come for the Sri Lankan government to act.
"The resolution's endorsement of a judicial mechanism with international participation is an important recognition of the need for an international role to ensure justice for victims.