Addressing a special session of the United Nations Human Rights Council, Michelle Bachelet voiced deep concern about the "high level of civilian fatalities and injuries" from the attacks on Gaza.
"If found to be indiscriminate and disproportionate in their impact on civilians and civilian objects, such attacks may constitute war crimes," she warned.
She also said her office had "not seen evidence" that the buildings targeted in Gaza, including residential homes, medical facilities and media offices, were "hosting armed groups or being used for military purposes", as claimed by Israel.
Bachelet also stressed that rockets fired by Hamas were "indiscriminate and fail to distinguish between military and civilian objects," and were thereby "a clear violation of international humanitarian law."
The UN High Commissioner for Human Rights made her statement at the start of a special one-day council session focused on the recent flare-up of violence.
Before a truce took hold last Friday, Israeli air strikes and artillery fire on Gaza killed 254 Palestinians, including 66 children, and wounded more than 1,900 people in 11 days of conflict, the health ministry in Gaza says.
Rocket and other fire from Gaza claimed 12 lives in Israel, including one child and an Arab-Israeli teenager, medics say. Some 357 people in Israel were wounded.
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