The United Nations called Tuesday for “a clear, transparent and credible investigation” of mass graves uncovered at two major hospitals in war-torn Gaza that were raided by Israeli troops.

Credible investigators must have access to the sites, U.N. spokesman Stephane Dujarric told reporters, and added that more journalists need to be able to work safely in Gaza to report on the facts.

Earlier Tuesday, U.N. human rights chief Volker Türk said he was “horrified” by the destruction of the Shifa medical center in Gaza City and Nasser Hospital in the southern city of Khan Younis as well as the reported discovery of mass graves in and around the facilities after the Israelis left.

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He called for independent and transparent investigations into the deaths, saying that “given the prevailing climate of impunity, this should include international investigators.”

“Hospitals are entitled to very special protection under international humanitarian law,” Türk said. “And the intentional killing of civilians, detainees and others who are ‘hors de combat’ (incapable of engaging in combat) is a war crime.”

U.S. State Department spokesman Vedant Patel on Tuesday called the reports of mass graves at the hospitals “incredibly troubling” and said U.S. officials have asked the Israeli government for information.

The Israeli military said its forces exhumed bodies that Palestinians had buried earlier as part of its search for the remains of hostages captured by Hamas during its Oct. 7 attack that triggered the war. The military said bodies were examined in a respectful manner and those not belonging to Israeli hostages were returned to their place.

The Israeli military says it killed or detained hundreds of militants who had taken shelter inside the two hospital complexes, claims that could not be independently verified.

The Palestinian civil defense in the Gaza Strip said Monday that it had uncovered 283 bodies from a temporary burial ground inside the main hospital in Khan Younis that was built when Israeli forces were besieging the facility last month. At the time, people were not able to bury the dead in a cemetery and dug graves in the hospital yard, the group said.

The civil defense said some of the bodies were of people killed during the hospital siege. Others were killed when Israeli forces raided the hospital.

Palestinian health officials say the hospital raids have destroyed Gaza’s health sector as it tries to cope with the mounting toll from over six months of war.

The issue of who could or should conduct an investigation remains in question.

For the United Nations to conduct an investigation, one of its major bodies would have to authorize it, Dujarric said.

“I think it’s not for anyone to prejudge the results or who would do it,” he said. “I think it needs to be an investigation where there is access and there is credibility.”

The chief prosecutor of the International Criminal Court, Karim Khan, said after visiting Israel and the West Bank in December that a probe by the court into possible crimes by Hamas militants and Israeli forces “is a priority for my office.”

The discovery of the graves “is another reason why we need a cease-fire, why we need to see an end to this conflict, why we need to see greater access for humanitarians, for humanitarian goods, greater protection for hospitals” and for the release of Israeli hostages, Dujarric said Monday.

In the Hamas attack that launched the war, militants killed about 1,200 people, mostly civilians, and abducted around 250 hostages. Israel says the militants are still holding around 100 hostages and the remains of more than 30 others.

In response, Israel’s air and ground offensive in Gaza, aimed at eliminating Hamas, has killed more than 34,000 Palestinians, according to local health officials, around two-thirds of them children and women. It has devastated Gaza’s two largest cities, created a humanitarian crisis and led around 80% of the territory’s population to flee to other parts of the besieged coastal enclave.