Mr Falk throws doubt on the legality of Israel's mission in Gaza
UN human rights investigators have questioned the legality of Israel's Gaza offensive in a wide-ranging report to the UN Human Rights Council.
One investigator, Richard Falk, asked how a military assault with modern weapons could have been made "against an essentially defenceless society".
The report found civilians appeared to "have taken the brunt of the attacks" with schools and clinics also hit.
Israel has accused Council members of seeking to "demonise" it.
Many international organisations have raised concerns about possible war crimes during the three-week offensive, which ended on 18 January.
An Israeli rights group has also criticised Israel for hitting medics and impeding medical evacuations.
The Israeli military says it is investigating specific claims of abuses and argues that it did its utmost to protect civilians during a conflict in which militants operated from populated civilian areas.
Because Mr Falk, a UN human rights investigator, was unable to enter the Palestinian territories, his latest findings focus on the legality of Israel's January operation in Gaza in general, rather than on specific cases or claims that disproportionate force was used.
Mr Falk argued that in order to determine if the war was legal, it was necessary to assess whether Israeli forces could differentiate between civilian and military targets in Gaza.
"If it is not possible to do so, then launching the attacks is inherently unlawful, and would seem to constitute a war crime of the greatest magnitude under international law," he said in Geneva.
"The overall ratio of deaths, 1,434 on the Palestinian side, 13 on the Israeli side, is suggestive of the one-sidedness of the military encounter," he added.
Gaza's borders were closed, he stressed, so civilians were unable to flee the fighting.
Mr Falk called for an independent inquiry to examine possible war crimes committed by both Israel and Hamas.
He also suggested that the Israeli blockade of Gaza was in violation of the Geneva Conventions and must be lifted.
Radhika Coomaraswamy, the UN secretary general's special representative for children and armed conflict, cited a long series of incidents to back her charges, in her report to the UN Human Rights Council.
In one, she said, Israeli soldiers shot a father after ordering him out of his house and then opened fire into the room where the rest of the family was sheltering, wounding the mother and three brothers and killing a fourth.
Israel's ambassador to the UN Human Rights Council, Aharon Leshno Yar, said the report "wilfully" ignored and downplayed terrorist and other threats faced by Israel
The 43-page document was, he added, part of a pattern of "demonising Israel" in the Council.
Israeli authorities denied entry to Mr Falk last December, when he attempted to conduct his regular investigative mission to the Palestinian territories.
Israel was angered by a series of comments he had made accusing it of war crimes and comparing its actions in Gaza to Nazi Germany in World War II.
On Monday, the Israeli organisation Physicians for Human Rights released a report saying Israel had violated international law and ethics codes during the Gaza operation.
It accused Israeli forces of "attacks on medical personnel, damage to medical facilities and indiscriminate attacks on civilians not involved in the fighting".