The commission, which was appointed by Mahinda Rajapaksa, Sri Lanka's president, will make recommendations on how to ensure that the country does not return to conflict, its chairman said at the launch event on Wednesday.

C.R. de Silva said in his opening remarks that the commission would "consolidate the military victory by addressing the root causes of the conflict and establish national integriy and reconciliation".

'Lacking credibility'

But human rights groups have said the commission is simply aimed at deflecting calls for an international investigation.

"It's pretty likely that this commission will be a whitewash"

Yolanda Foster, Sri Lanka expert, Amnesty International

"There had been previous commissions and they have not borne fruit," Meenakshi Ganguly, Human Rights Watch's South Asia director, said.

"We are not convinced of the commitment of the government of Sri Lanka to ensure accountability."

Yolanda Foster, Amnesty International's Sri Lanka expert, told Al Jazeera that the country suffered from a "climate of impunity" which had compromised previous domestic probes in to human rights abuses.

"Sri Lanka has been plagued by human rights violations from a number of actors. There is no space in the justice sector to address this," she said.

"It's pretty likely that this commission will be a whitewash, which is why we are calling for an independent international inquiry."

Meanwhile, an open letter to Hillary Clinton, the US secretary of state, from 57 US politicians, said that the Sri Lankan panel "lacked the needed credibility" to investigate such serious allegations. 

"There is mounting evidence that suggests both parties in the conflict committed severe human rights violations during the conflict," the letter said.

"We urge you to call for a robust and independent international investigation that would finally clarify the events that occurred during the conflict and provide the foundation for a sustainable peace in Sri Lanka."

Civilians killed

Government forces are accused of firing shells into civilian areas, attacking hospitals, and bombing food distribution points in the Tamil-majority areas where the fighting was heaviest.

The Sri Lankan government has dismissed the allegations, insisting that no civilians were killed in the final offensive, despite a mounting body of evidence to the contrary. 

The UN, which released a report estimating that 7,000 Tamil civilians died in the offensive, set up a three-member panel in June to advise Ban Ki-Moon, the UN secretary general, on how to ensure that the alleged war-crimes did not go unpunished.

The move sparked outrage in Colombo, where the government refused to co-operate with the investigation and said it was an infringement on Sri Lanka's sovereignty.

The next formal hearing of the commission is scheduled for August 17, but not all the proceedings will be made public.