"There are strong reasons to believe that both sides have grossly disregarded the fundamental principle of the inviolability of civilians," Pillay told the UN Human Rights Council as it opened a special session on Sri Lanka.
"An independent and credible international investigation into recent events should be dispatched to ascertain the occurrence, nature and scale of violations of international human rights and international humanitarian law, as well as specific responsibilities," she added.
Pillay reiterated her concern about allegations that Tamil Tiger rebels prevented civilians from fleeing the combat zone in recent weeks and effectively used them as human shields.
But in a joint statement to the Council, UN human rights experts went a step further.
"It is clear that the Liberation Tigers of Tamil Eelam has acted in flagrant violation of the applicable norms of international law by using civilians as human shields or in preventing them from leaving the conflict areas," they said.