The UN human rights office says it is “profoundly shocked” at the impact on civilians of the Israeli operation in central Gaza that rescued four hostages held by Hamas.
Palestinian health officials said hundreds of people were killed and injured in the densely-populated Nuseirat refugee camp on Saturday. Israel's military said fewer than 100 were killed.
UN spokesman Jeremy Laurence said the action by Israeli forces “seriously calls into question whether the principles of distinction, proportionality and precaution... were respected” and could amount to war crimes.
He also said Palestinian armed groups could face war crimes charges for continuing to hold hostages in built-up areas and "putting the lives of Palestinian civilians, as well as the hostages themselves, at added risk”.
Israel’s mission to the UN in Geneva accused the UN human rights office of “slander”.
“The toll of this war on civilians is first and foremost the product of Hamas’s deliberate strategy to maximise civilian harm,” a statement said.
“Those who continue to shield Hamas terrorists, including [the UN human rights office], are complicit in the suffering of Palestinians and Israelis alike.”
The Israeli military has repeatedly said it operates in accordance with international law.
There was no immediate comment by Hamas.
The four hostages freed on Saturday - Noa Argamani, Almog Meir, Andrey Kozlov and Shlomi Ziv - were held in two apartment buildings about 200m (656ft) apart in Nuseirat - a historic, urban refugee camp which has seen an influx of displaced people since the start of the war.
According to the Israel Defense Forces (IDF), the Israeli commandos took Ms Argamani’s Hamas guards by surprise and quickly killed them. But the simultaneous move to free Mr Ziv, Mr Kozlov and Mr Meir from the second building sparked a fierce gun battle with their guards, during which a senior Israeli police officer was fatally wounded.
As the commandos evacuated to the coast, they came under fire from fighters armed with machine-guns and rocket-propelled grenades, the IDF said. In response, Israeli aircraft, artillery and naval vessels carried out intense strikes on the area.
Gaza’s Hamas-run health ministry said 274 Palestinians were killed and 698 others were injured during the operation. Its figures do not differentiate between civilians and combatants.
The Hamas-run Government Media Office reported that 64 children, 57 women and 37 elderly people were among the dead.
Médecins Sans Frontières (MSF) said its teams, along with medical staff at al-Aqsa hospital in the nearby town of Deir al-Balah and Nasser hospital in the southern city of Khan Younis, treated hundreds of severely injured patients, many of whom were women and children.
The charity also quoted one of its Palestinian doctors, Dr Hazem Maloh, as saying that dozens of men, women and children were killed, including his neighbours, friends or relatives.
The director of al-Awda hospital in Nuseirat told BBC Arabic’s Gaza Today programme that 142 dead and 250 injured people were brought to the hospital on Saturday, and that almost a quarter of the fatalities were women and children.
A senior official from the UN agency for Palestinian refugees (Unrwa), meanwhile, said its health centre in Nuseirat treated more than 125 injured people.
The IDF's chief spokesman said it was aware of "under 100" Palestinian casualties and that it did not know how many of them were "terrorists".
Several Nuseirat residents interviewed for BBC Arabic’s Gaza Today programme said they were reluctant to leave the camp in the wake of the operation because nowhere else in Gaza was safe.
"They hit the house opposite us with missiles. I thought my family and I would surely die. Another missile also hit the door of our house. And of course there was widespread destruction," Khalil al-Tahran said.
"But where should we flee to?"
Another man, Salem Wali al-Dasht, said: "Death here will not be different from death [somewhere else]... that is why we decided to remain where we are."
An Israeli doctor who treated the hostages following their rescue said they had suffered a "harsh experience, with a lot of abuse, almost every day".
"Every hour, both physical, mental and other types, and that is something that is beyond comprehension," Dr Itai Pessach of the Sheba Medical Centre in Ramat Gan told CNN.
He also said their time in captivity had "left a significant mark on their health".
This was due to a "combination of the psychological stress, malnutrition... medical neglect, being limited [in] space, not seeing the sun”, he explained.
UN spokesman Mr Laurence noted that the UN human rights office’s ability to verify reports of casualties during the hostage rescue operation was limited because of access constraints, but that it still had "reliable" contacts on the ground.
He also said that UN High Commissioner for Human Rights Volker Türk welcomed the UN Security Council resolution endorsing a proposed ceasefire and hostage release deal between Israel and Hamas, which was outlined by the US last month.
US Secretary of State Antony Blinken said Israel’s prime minister had "reaffirmed his commitment" to the plan at a meeting on Monday.
Hamas has yet to accept it, but Mr Blinken said a statement from the group welcoming the UN resolution was a "hopeful sign".
The Israeli military launched a campaign in Gaza to destroy Hamas in response to an unprecedented cross-border attack on southern Israel on 7 October, during which about 1,200 people were killed and 251 others were taken hostage.
More than 37,120 people have been killed in Gaza since then, according to the Hamas-run health ministry.
A deal agreed in November saw Hamas release 105 of the hostages in return for a week-long ceasefire and some 240 Palestinian prisoners in Israeli jails. Israel says 116 hostages are still being held, 41 of whom are presumed dead.