Bachelet said her office had verified the deaths of 270 Palestinians in Gaza, the West Bank and East Jerusalem, including 68 children, during violence this month. Most were killed in Hamas-controlled Gaza, where Israel fought militants for 11 days, ending in a ceasefire.
Hamas rockets had killed 10 Israelis and residents.
She was addressing a special session of the U.N. Human Rights Council, held at the request of Muslim states who have asked the forum to set up a commission of inquiry to investigate possible crimes and establish command responsibility.
"If found to be indiscriminate and disproportionate, such attacks might constitute war crimes," Bachelet told the 47-member forum. She also urged Hamas to refrain from firing indiscriminate rockets on Israel.
The U.N. Special Rapporteur on the situation of human rights in the Palestinian territories, Michael Lynk told the Council that events of the lasts weeks, "haunts the conscience of the world" and warned on the situation in Gaza, which he called the "world's largest open-air prison."
Meirav Eilon Shahar, Israel's ambassador to the U.N. in Geneva, denounced Hamas as a "jihadist, genocidal, terrorist organization" and accused it of using Palestinian civilians as human shields to conceal its rockets.