International Criminal Court (ICC) prosecutors have reportedly gathered testimony from staff of two major hospitals in the Gaza Strip, in what is believed to be the first confirmation that ICC investigators are speaking to medical workers about possible crimes during Israel’s nearly seven-month war on the besieged territory.

The sources, who asked not to be identified due to the sensitivity of the subject, told Reuters news agency that the investigators had interviewed staff who had worked at al-Shifa Hospital and Nasser Hospital, on the grounds of which Palestinian officials say they have discovered mass graves following the withdrawal of Israeli troops.

The sources declined to provide more details, citing concerns about the safety of potential witnesses, Reuters reported on Monday. One of the sources said that events surrounding the hospitals could become part of the investigation by the ICC, which hears criminal cases against individuals for war crimes, crimes against humanity, genocide and aggression.

Last week, the United Nations human rights office said it was “horrified” by reports of mass graves found at al-Shifa and Nasser after Israeli sieges and raids that damaged the facilities, noting the “special protection” awarded to medical facilities under international law.

The two sources quoted by Reuters were not able to say whether such graves formed part of any questioning. The ICC’s office of the prosecutor refused to comment on operational matters in ongoing investigations citing the need to ensure the safety of victims and witnesses.

Gaza’s civil defence agency said on Thursday the graves found in the two hospitals contained 392 bodies, including those of women, children and the elderly. Ten of the bodies were found with bound hands while others still had medical tubes attached to them, indicating they may have been buried alive, said civil defence member Mohammed Mughier.

The Israeli army said claims it had buried Palestinian bodies were “baseless”, without directly addressing allegations that Israeli troops were behind the killings. It said “corpses buried by Palestinians” had been examined by Israeli troops searching for hostages and then “returned to their place”.

During the war, the two hospitals have been high-profile Israeli targets – surrounded, besieged and stormed by Israeli forces who accuse Hamas of using them for military purposes, which Hamas and medical staff deny. Israel denies carrying out war crimes, including in or around Gaza hospitals.

‘Walls are closing in’

The ICC has said it is investigating both sides in the conflict, including both the Hamas-led October 7 attack in southern Israel that killed more than 1,100 people and the subsequent Israeli war on Gaza, which has killed more than 34,500.

Israel is not a member of the ICC, while the Palestinian territories were admitted as a member state in 2015. The ICC says this gives it jurisdiction over actions by anyone including Israeli soldiers in the Palestinian territories, and by Palestinians anywhere, including on Israeli territory. Israel does not recognise any ICC jurisdiction over its citizens.

Any ICC criminal case would be separate from a case in the International Court of Justice, which was brought by South Africa and accuses Israel of genocide in Gaza, which Israel denies. The ICJ, also based in The Hague, hears lawsuits between states, while the ICC hears criminal cases against individuals.

Israel’s Prime Minister Benjamin Netanyahu said on Friday any ICC move would not affect Israel’s actions but would “set a dangerous precedent that threatens soldiers and public figures”.

“Under my leadership, Israel will never accept any attempt by the International Criminal Court in the Hague to undermine its basic right to defend itself,” he wrote on Telegram.

Salman Shaikh, a former Middle East peace envoy with the UN, said Israel’s conduct in Gaza is increasingly under the spotlight worldwide and international courts are expected to hold Israeli officials accountable.

“The walls are closing in. International law has to be administered. Otherwise, we will see Western countries effectively cannibalising international law and the rules-based international order, which they themselves set up after the horrors of the second world war,” Shaikh told Al Jazeera.

“We have ample evidence and those who have committed grave violations of international humanitarian law – whether they are Hamas or other Palestinian groups but also the Israeli army. This cannot continue.”

Western countries should be extremely concerned about their support for Israel during its war on Gaza, he added. “European capitals and Washington need to think really hard about their actions, including the supply of offensive weaponry to the Israeli army.”