GENEVA - A UN investigator defended a report Tuesday that accuses Israel and Palestinian militants of war crimes during their conflict in Gaza, an allegation Israel condemns and claims is the result of bias against the Jewish state.

U.S. officials criticized the report that came down heavily on its longtime ally and indicated they would block moves to refer the findings to the powerful UN Security Council.

"We disagree sharply with many of the report's assessments and its recommendations and believe it to be deeply flawed," U.S. Assistant Secretary of State Michael H. Posner said.

The report's lead author, former South African judge Richard Goldstone, said he and his team were disappointed by the criticism and rejected any suggestion the findings in 575-page document were politically motivated.

"We believe deeply in the rule of law, humanitarian law, human rights and the principle that in armed conflict civilians should to the greatest extent possible be protected from harm," Goldstone told the UN Human Rights Council in Geneva that commissioned the report.

Investigators were driven by a desire to hold accountable those on both sides who harmed civilians and failure to do so would undermine the Israeli-Palestinian peace process, Goldstone said.

The report into the Dec. 27-Jan. 18 conflict in which almost 1,400 people were killed -- most of them Palestinian civilians -- recommended that the UN Security Council in New York require both sides to show they are carrying out credible investigations into alleged abuses during the conflict.

UN investigators examined 36 incidents and interviewed dozens of Palestinian and Israeli witnesses in Gaza and Geneva. The incidents include one case in which Israeli forces allegedly shelled a house where soldiers had forced Palestinian civilians to assemble, and seven incidents in which civilians were shot while leaving their homes trying to run for safety.

On the Palestinian side, the report found that armed groups firing rockets into southern Israel from Gaza failed to distinguish between military targets and the civilian population, and collected several reports of Palestinians being held as human shields by militants.

Israeli Ambassador Aharon Leshno-Yaar described the report as "shameful," claiming it cherry-picked incidents for political effect, ignored Israel's right to self-defence, and provided "support and vindication for terrorist tactics."

"Israel is committed to fully examining every allegation of wrongdoing, not because of this report, but despite it," Leshno-Yaar said, noting that more than 100 investigations have been launched and 23 have resulted in criminal proceedings.

Human rights groups say that so far these are internal military probes.

"These are not independent, impartial investigations," said Amiram Gill of Physicians for Human Rights Israel. The group says only international pressure will force Israel and the Palestinians into setting up credible probes of alleged perpetrators of war crimes.

Goldstone said Israel's efforts so far were "pusillanimous," while investigations by the Palestinian group Hamas, which controls Gaza, were a "complete failure."

European and U.S. diplomats have made it clear that they object to a resolution by Arab and Muslim countries endorsing the report. Western countries want to keep the report in Geneva and avoid escalating it to the UN Security Council from where it might be passed to the International Criminal Court, the world's first permanent war crimes tribunal.

Posner said allowing the UN rights council to refer the report to New York would set a bad precedent.

"If this standard were applied in every conflict situation around the world where there are alleged violations, then the role of the Human Rights Council would be dramatically different," he said.

Posner's intervention was the first sour note from the U.S. in the council since it joined the body earlier this year as part of President Barack Obama's drive to engage the UN and the Arab world.

Goldstone said much depended on what next steps the international community decides to take on the report.

"It's all a matter of politics at this stage," he said.