Israel's recent barrage of deadly air strikes on Gaza might be war crimes, the UN rights chief said Thursday, adding she had seen no evidence the attacked buildings were used for military purposes.

Addressing a special session of the United Nations Human Rights Council, Michelle Bachelet voiced deep concern about the surge in deadly violence between Israelis and Palestinians earlier this month.

"Although Israel undertook a number of precautions, such as advance warning of attacks in some cases, air strikes in such densely populated areas resulted in a high level of civilian fatalities and injuries, as well as the widespread destruction of civilian infrastructure," the UN High Commissioner for Human Rights said.

"If found to be indiscriminate and disproportionate in their impact on civilians and civilian objects, such attacks may constitute war crimes."

Before a truce took hold last Friday, Israeli air strikes and artillery fire on Gaza killed 253 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.

Bachelet called on "Hamas and all armed groups to refrain from use of indiscriminate rockets and mortars, for which there must be accountability". 

"There is no doubt that Israel has the right to defend its citizens and residents," she said. "However, Palestinians have rights too. The same rights." 

The UN rights chief highlighted the scale of the destruction in Gaza.

"Although reportedly targeting members of armed groups and their military infrastructure, Israeli attacks resulted in extensive civilian deaths and injuries, as well as large-scale destruction and damage to civilian objects," she said.

She pointed out that governmental buildings, residential homes, international humanitarian organisations, medical facilities and media offices had been hit. 

"Despite Israel's claims that many of these buildings were hosting armed groups or being used for military purposes, we have not seen evidence in this regard," she said.

The 47-member council was debating a draft resolution to launch a broad, international investigation into violations surrounding the latest Gaza violence, but also of "systematic" abuses in the Palestinian territories and inside Israel.