UN human rights chief Navi Pillay on Thursday slammed what she said was Israel's "deliberate defiance" of international law during the Gaza conflict.
Pillay lambasted the country's attacks on homes, schools, hospitals and United Nations facilities which are sheltering 250,000 civilians in Gaza.
"There appears to be deliberate defiance of obligations that international law imposes on Israel," the South African told reporters.
Pillay said that repeated calls to respect the laws of war had gone unheeded during the latest crisis and previous spikes in the Israeli-Palestinian conflict.
"The same pattern of attacks is occurring now on homes, schools, hospitals, UN premises. None of this appears to me to be accidental," she said.
She spoke a day after Israeli shells slammed into a UN school in Jabalia refugee camp which was sheltering some 3,300 homeless Gazans, killing 16 people.
Israel has accused Palestinian Islamists Hamas and other militants of hiding out among the civilian population and using UN facilities and other sites to store weapons and launch rocket attacks.
Pillay said that under international law, civilian facilities should not be attacked, but can lose their protected status if used for military purposes.
Even then, she said, due warning must be given before an attack, in order to allow civilians to be evacuated.
"It is completely unconscionable that the proportionality and precaution that international law requires is being ignored," said Pillay.
She also criticised Israel's strikes on Gaza's power plant, as well as water and sewerage systems.
Last week, the UN Human Rights Council voted to open an inquiry into the Gaza offensive, despite fierce opposition from Israel and the United States.
"We cannot allow impunity. We cannot allow this lack of accountability to go on," Pillay said on Thursday, calling into question domestic investigations by Israel into abuses.
"I join the world in condemning the aggression that is taking place in Gaza, and particularly the killing of civilians. This is wrong and it will always be wrong," she added.
As of Wednesday, the 24th day of the Gaza conflict, 1,364 Palestinians had been killed -- three-quarters of them civilians, Pillay said.
Fifty-six Israeli soldiers have also died, while cross-border rocket fire has killed two Israeli civilians and a Thai migrant worker.
The UN human rights chief has repeatedly condemned the actions not only of Israel but also the indiscriminate Palestinian rocket attacks on Israeli communities.