Amnesty International is urging the United Nations to establish a credible investigation into war crimes allegedly committed in Sri Lanka during the final phases of the country's civil war in 2009.

The London-based watchdog said Wednesday that Sri Lanka's official inquiry into the matter is “seriously flawed” and “falls short of international standards.”

The country's 26-year civil war ended in 2009 when government forces defeated Tamil Tiger rebels. Both sides are accused of committing war crimes, but Amnesty says most civilian casualties in the final phases of the war were caused by government shelling.

In May 2010, Sri Lankan President Mahinda Rajapaksa announced a national commission to investigate the allegations. The “Lessons Learnt and Reconciliation Commission” is set to report its findings to the government this fall.

But Amnesty says the commission is unlikely to deliver justice for the “tens of thousands of victims of war crimes,” saying that the investigation has so far made no recommendation for bringing individuals to justice, and has failed to protect Sri Lankans who offer testimony.

The U.N. Human Rights Council is expected to discuss Sri Lanka at a three-week session starting on Monday. The U.N. panel previously labeled the commission “inadequate.”

Sri Lanka's government has denied the allegations of war crimes, saying that civilian deaths were impossible to avoid altogether, given the magnitude of the fighting and the “ruthlessness” of the opponent.