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 8 July 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,” Ms Pillay said.
However Israel, which claims the UN Human Rights Council is biased, is unlikely to co-operate with any authorised UN investigation, the BBC’s Imogen Foulkes in Geneva reports.
Israel’s Justice Minister, Tzipi Livni, said her country was acting according to international law.
“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,” she told Israel radio.
Ms Livni also described the UN Human Rights Council as an “anti-Israel” body.
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.
Kyung-wha Kang, the assistant secretary-general at the UN Office for the Coordination of Humanitarian Affairs, said civilians in Gaza had no safe to place to go “as 44% of the land has been declared a ‘no-go zone’ by the Israeli army”.
“Families are taking the heart-wrenching decision to split to different locations – mother and son to one; father and daughter to another – hoping to maximise 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. At least five people died in an air strike in the town overnight. An Israeli soldier was also killed.
Witnesses say around 5,000 Palestinians, some waving white flags, are fleeing in a state of panic following a ground incursion by Israeli troops, the BBC’s Paul Adams in Gaza reports.
A Palestinian woman whom the BBC filmed being pulled from the rubble of a Gaza blast on Sunday also died from her injuries, her doctor said. Ten of her relatives were killed in the blast.
At the scene: The BBC’s Bethany Bell at Ben Gurion airport
A number of airlines are still operating in and out of Tel Aviv. Many of the passengers we’ve spoken to appear relaxed about possible security risks and delays. But there hasn’t been this much disruption at Ben Gurion Airport since the First Gulf War in 1991.
This is a major transport hub and many Israelis are concerned that the decision by some airlines to suspend flights hands Hamas a symbolic victory. A headline in the Haaretz newspaper put it succinctly: “Frightening airlines into suspending flights to Israel, Hamas scores a major achievement.”
Israel, the paper remarks, can cope with a “temporary blip”. But a longer suspension, it says, could hurt the economy and damage the country’s self-confidence. Israel’s leaders say Ben Gurion airport is safe. They are putting pressure on the United States for flights to resume as quickly as possible.
Meanwhile US Secretary of State John Kerry arrived in Israel on Wednesday to try to help negotiate a truce.
Mr Kerry flew by military plane to Ben Gurion airport near Tel Aviv. 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.
Referring to a 16 July Israeli air strike that killed children playing on a beach in Gaza, Ms 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, accusing Hamas and other armed Palestinian groups of failing to observe “the principles of distinction and precaution”.
Despite her condemnation of Hamas attacks on Israel, Ms Pillay clearly views Israel’s actions in Gaza as disproportionate, our correspondent says.
Earlier, Israeli Prime Minister Benjamin Netanyahu said Hamas should be held accountable for rejecting an Egyptian ceasefire proposal.
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.
In 2011, the report’s author, South African judge Richard Goldstone, said that new accounts indicated Israel had not deliberately targeted civilians.