Richard Falk, UN special rapporteur on human rights in the Palestinian territories, said the Geneva Conventions required warring forces to distinguish between military targets and surrounding civilians.
"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," Falk said.
"On the basis of the preliminary evidence available, there is reason to reach this conclusion," he wrote in an annual report submitted to the United Nations Human Rights Council.
Falk called for an independent experts group to be set up to probe possible war crimes committed by both Israeli forces and Hamas.
Violations included Israel's alleged "targeting of schools, mosques and ambulances" during the December 27-January 18 offensive and its use of weapons including white phosphorus, as well as Hamas firing of rockets at civilian targets in southern Israel.
Falk said that Israel's blockade of the coastal strip of 1.5 million people violated the Geneva Conventions, which he said suggested further war crimes and possibly crimes against humanity.
The aggression was not legally justified and may represent a "crime against peace" - a principle established at the Nuremberg trials of Nazi criminals, according to the American law professor who serves as the Human Rights Council's independent investigator.
He further suggested that the Security Council might set up an ad hoc criminal tribunal to establish accountability for war crimes in Gaza, noting Israel has not signed the Rome statutes establishing the International Criminal Court.
Rights group names 1,417 Gaza war dead; Israel disputes toll
A Palestinian human rights group has released the names of 1,417 Gazans it says were killed in Israel's recent war on the Palestinian territory's Hamas rulers.
The Palestinian Center for Human Rights said Thursday that of those killed, 926 were civilians, 236 were combatants and 255 were members of the Palestinian security forces.
Most of the policemen were killed in a series of Israeli bombing attacks on Hamas security compounds on December 27, the first day of the war.
The group says it has investigated every civilian death. The list is posted on the center's Web site.
Israeli government spokesman Mark Regev disputes the findings. He says Israel is working on its own list and contends that most of those killed were combatants or legitimate targets.
Thirteen Israelis were killed during the 22-day Gaza operation.