Navi Pillay told an emergency debate at the UN Human Rights Council in Geneva that Israel’s military offensive had not done enough to protect civilians.
She also condemned Hamas for “indiscriminate attacks” on Israel.
Israel launched its offensive on July 8 with the declared objective of stopping rocket fire from Gaza.
“There seems to be a strong possibility that international law has been violated, in a manner that could amount to war crimes,” Navi Pillay said.
Israel’s Justice Minister Tzipi Livni described the UN Human Rights Council as an “anti-Israel” body, Reuters news agency reported.
At least 649 Palestinians and 31 Israelis have been killed in the past 15 days of fighting, officials say.
A foreign worker in southern Israel was also killed by a rocket fired from Gaza on Wednesday, police said.
The UN says about 74% of those killed in Gaza are civilians, with medical clinics among the facilities hit by air strikes.
“Civilians in Gaza have no safe place to go as 44% of the land has been declared a ‘no-go zone’ by the Israeli army,” the assistant secretary-general at the UN Office for the Coordination of Humanitarian Affairs said.
“Families are taking the heart-wrenching decision to split to different locations – mother and son to one; father and daughter to another – hoping to maximize the chance one part of the family survives.”
There was heavy fighting in the town of Khan Younis in the southern Gaza Strip on Wednesday.
Eyewitnesses say around 5,000 Palestinians, some waving white flags, are fleeing in a state of panic following a ground incursion by Israeli troops.
Meanwhile US Secretary of State John Kerry arrived in Israel on Wednesday to try to help negotiate a truce.
“We have certainly made some steps forward. There is still work to be done,” John Kerry said shortly after arriving.
John Kerry flew by military plane to Ben Gurion airport near Tel Aviv, as several US and European airlines continue to suspend civilian flights into Israel after a rocket from Gaza landed near the airport.
German airline Lufthansa announced on Wednesday it would extend the ban for another 24 hours.
At least five people died in an air strike in Khan Younis overnight. An Israeli soldier was also killed.
Earlier, Israeli PM Benjamin Netanyahu said Hamas should be held accountable for rejecting an Egyptian ceasefire proposal.
Referring to a July 16 Israeli air strike that killed children playing on a beach in Gaza, Navi Pillay said “the disregard for international humanitarian law and for the right to life was shockingly evident”.
She also condemned rocket attacks from Gaza into Israel.
“The principles of distinction and precaution are clearly not being observed during such indiscriminate attacks on civilian areas by Hamas and other armed Palestinian groups,” she told the UN Human Rights Council.
Despite her condemnation of Hamas attacks on Israel, Navi Pillay clearly views Israel’s actions in Gaza as disproportionate.
“Israel is acting according to international law,” Tzipi Livni said.
“It is regrettable civilians are killed, but when we call on them to vacate and Hamas calls on them to stay, then that is what happens.”
A 2009 UN human rights report said that Israel’s military and Hamas had each committed potential war crimes during Israel’s 2008-2009 offensive in Gaza.
The Goldstone report was rejected by Israel and the US as biased and flawed.
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