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UN officials said they were horrified at the reports of mass graves at a major hospital in the Gazan city of Khan Younis, where local authorities said they found bodies stripped naked with their hands tied.
Civil defence teams have reported finding hundreds of bodies at Nasser Hospital this week since it was abandoned by Israeli troops, who left the medical complex badly damaged after raiding it.
Yamen Abu Suleiman, director of civil defence in Khan Younis, told CNN 35 bodies were discovered on Tuesday, bringing the total to 310.
“We do not know if they were buried alive or executed. Most of the bodies are decomposed,” he said.
Palestinian news agency Wafa reported people of various ages were found in the grave, which it said was created by the Israeli army before it withdrew from the hospital last month.
The UN voiced its alarm at the reports, with the High Commissioner for Human Rights, Volker Turk, saying he was "horrified".
He called for an independent investigation into the killing of civilians at Nasser Hospital and what was Gaza's largest medical complex, Al Shifa Hospital.
“The intentional killing of civilians, detainees and others who are 'hors de combat' is a war crime,” Mr Turk said.
“We feel the need to raise the alarm because clearly there have been multiple bodies discovered,” Mr Turk's representative Ravina Shamdasani said.
“Some of them had their hands tied, which indicates serious violations of international human rights law and international humanitarian law, and these need to be subjected to further investigations,” she said.
Palestinians searching for relatives at Nasser Hospital said their bodies had been moved from their original burial site after previous raids on the hospital.
Israel denies claims
The Israeli military said claims it had buried bodies were “baseless and unfounded”.
It said forces searching for Israeli hostages had examined bodies previously buried by Palestinians near Nasser Hospital and had returned the bodies to where they were buried after examination.
“The examination was conducted in a careful manner and exclusively in places where intelligence indicated the possible presence of hostages. The examination was carried out respectfully while maintaining the dignity of the deceased,” it said.
The US called the scenes "deeply concerning" but the White House said it was "not in a position to confirm" their veracity.
"We will certainly talk to our Israeli counterparts to see what they know but I can't go into more detail," National Security Council spokesman John Kirby said.
The Israeli army launched weeks-long raids on Nasser Hospital, killing and displacing medical staff and patients. The army had claimed that it was one of the hospitals were being used as command centres by Hamas.
Hospital authorities have denied the claims.
The Wafa report claimed that the “vast majority” of civilians killed in Gaza had been buried in mass graves, created by Israeli bulldozers before the army withdrew from particular areas.
It said about 2,000 people are missing across various parts of Gaza, including hundreds in Khan Younis.
In February, the enclave's Health Ministry said Nasser had been turned into a “military site” by Israel as wheelchair-bound patients and sick Palestinians were seen fleeing to the nearby city of Rafah.
Rafah warning
The UN's rights chief, Mr Turk, also warned against an Israeli offensive on Rafah in southern Gaza, the last refuge for more than a million Palestinians displaced from other areas in the enclave.
“The world’s leaders stand united on the imperative of protecting the civilian population trapped in Rafah,” Mr Turk said.
He also condemned an air strike on an apartment building in the Tal Al Sultan area of Rafah, which killed nine Palestinians, and a strike on a refugee camp that killed four people, including a girl and a pregnant woman.
“The latest images of a premature child taken from the womb of her dying mother, of the adjacent two houses where 15 children and five women were killed, this is beyond warfare,” said Mr Turk.
The mounting death toll comes as the war enters its 200th day.
About 34,200 Palestinians have been killed and 77,143 wounded since the war began on October 7, caused by the Hamas attack on southern Israel that killed about 1,200 people, with 240 taken hostage by the militant group.
While ground forces largely withdrew from southern Gaza earlier this month, air strikes and shelling continue across the enclave, according to Palestinian media.
Beaches in Al Zawaida, Deir Al Balah, and Nuseirat were all bombed at dawn on Tuesday as “intense raids” were reported across the north, the official Wafa news agency reported.
Intense air strikes were also reported in Khan Younis.
Also on Tuesday, a Palestinian was killed in the occupied West Bank city of Jericho amid army raids on the city and its surrounding refugee camps.
Shadi Issa Galaita, 46, was shot dead while two others, including a child, were wounded, according to Wafa.
