The UN human rights office cited forcible transfer, collective punishment and the taking of hostages as the war continued into its 21st day.

"We are concerned that war crimes are being committed. We are concerned about the collective punishment of Gazans in response to the atrocious attacks by Hamas, which also amounted to war crimes," spokeswoman Ravina Shamdasani told a press conference in Geneva.

She said that it was for an independent court of law to qualify whether war crimes had been committed.

Israel has heavily bombarded Gaza since Hamas gunmen stormed across the border on October 7, killing 1,400 people, mostly civilians, kidnapping more than 220 others, according to Israeli officials.

The health ministry in the Hamas-run Gaza Strip said the strikes have killed more than 7,000 people, mainly civilians and many of them children.

Israel has cut supplies of food, water and power to Gaza, notably blocking all deliveries of fuel saying it would be exploited by Hamas to manufacture weapons and explosives.

Israel's army has called on people in the north of the Gaza Strip -- nearly half of its 2.4-million population -- to head south ahead of an expected ground offensive.

- 'Nowhere is safe' -

"Nowhere is safe in Gaza. Compelling people to evacuate in these circumstances... and while under a complete siege raises serious concerns over forcible transfer, which is a war crime," Shamdasani said.

"Israel's use of explosive weapons with wide-area effects in densely populated areas has caused extensive damage to civilian infrastructure and loss of civilian lives that, by all appearances, is difficult to reconcile with international humanitarian law," she added.

Shamdasani said a humanitarian catastrophe was unfolding for the people "locked inside Gaza who are being collectively punished. Collective punishment is a war crime. Israel's collective punishment of the entire population of Gaza must immediately cease."

She said indiscriminate attacks by Palestinian armed groups, including through the launching of unguided rockets into Israel, had to stop.

"They must immediately and unconditionally release all civilians who are captured and are still being held. The taking of hostages is also a war crime," the spokeswoman added.

In response, the Israeli mission in Geneva said international humanitarian law remained the Israeli military's "guide book", despite "Hamas brutality, and the shield the UN provides them with".

"Israel is at war with Hamas, a genocidal terrorist organisation. This is not a war with the Palestinians. This is not collective punishment," the mission said.

"Israel does its utmost to protect civilians and mitigate harm to them, including providing advance notice to civilians regarding safer zones, while Hamas deliberately puts them in harm's way."

"War crimes" were defined in the 1998 Rome Statute that established the International Criminal Court.

A war crime is a serious violation of international law against civilians and combatants during armed conflict, a "grave breach" of the 1949 Geneva Conventions that established a legal framework for war after the Nuremberg tribunals of top Nazis.