The UN's main human rights watchdog has begun a debate on a damning report into Israel's military operation against Gaza eight months ago.
It is seen as a test of US engagement with the Human Rights Council, which was shunned by President George W Bush.
The US, which is Israel's main ally, has criticised elements of the report.
The report, widely lauded by human rights groups, accuses both Israel and its militant Palestinian adversary Hamas of war crimes in the campaign.
Presenting the full version of the report, lead investigator Richard Goldstone told delegates that a lack of accountability for war crimes had reached "crisis point" in the Middle East and undermined any hope for peace in the region.
He rejected what he called a "barrage of criticism" about his findings and public attacks against the members of his mission.
"We will not address these attacks as we believe that the answers to those who have criticised us are in the findings of the report," he said.
The Human Rights Council was founded three years ago, after criticism of its predecessor that it turned a blind eye to many human rights abuses while having an in-built anti-Israel bias.
The Bush administration took no part in the new body, but the Obama administration sought a seat on the council after it came to power in January.
State department legal adviser Harold Koh is quoted describing US participation at the Geneva-based council as "an experiment".
"We think that this is a forum... where those three principles, engagement, universality and the truth, can be explored," Mr Koh briefed journalists ahead of the debate.
The 574-page report was written by a four-judge commission led by South African judge Richard Goldstone.
It accused both the Israeli army and Palestinian militants of deliberately terrorising and killing civilians on the other side.
It urged the UN Security Council to refer allegations to the International Criminal Court (ICC) if either side failed to investigate and prosecute suspects.
After publication of an advance version report on 15 September, the US said it would "carefully review" the document, but had "serious concerns" about the commission's mandate, which was set before the US joined the Human Rights Council.
It also rejected recommendations regarding the US Security Council, saying it should not even discuss the matter.
"The appropriate venue for this report to be considered is the Human Rights Council," said UN envoy Susan Rice.
Israel refused to co-operate with investigations and has rejected the findings as "flawed" and "biased".
Human Rights Watch, one of a number of NGOs that endorsed the report, urged the administration to reverse its position.
"The Obama administration cannot demand accountability for serious violations in places like Sudan and Congo but let allies like Israel go free," said HRW's Sarah Leah Whitson.
The enquiry found evidence "indicating serious violations of international human rights and humanitarian law were committed by Israel during the Gaza conflict".
Israel's operations, the document states, "were carefully planned in all their phases as a deliberately disproportionate attack designed to punish, humiliate and terrorise a civilian population".
The report also found evidence Palestinian groups committed war crimes, and possibly crimes against humanity, in repeated indiscriminate rocket and mortars attacks on Israel.
Hamas called the report "political, biased and dishonest" as it put people "who resist" crimes "on the same level as those who perpetrate" them.
The Israeli military has carried out more than 100 investigations into allegations of abuses by in Gaza. Most were dismissed as "baseless" but 23 criminal investigations are still ongoing.
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