Antonio Guterres says it is ‘imperative’ that independent investigators are allowed the immediate access to mass graves in Gaza to determine the circumstances behind the deaths.

United Nations chief Antonio Guterres has demanded an independent investigation into reports by Palestinian authorities of mass graves discovered in Gaza after bodies with hands tied or medical tubes attached to them were found in multiple sites, including two hospitals.

“It is imperative that independent international investigators with forensic expertise are allowed the immediate access to the sites of these mass graves to establish the precise circumstances under which the Palestinians lost their lives and were buried or reburied,” Guterres said on Tuesday.

The UN secretary-general also urged Israel against invading southern Gaza’s city of Rafah, the last refuge for more than 1.4 million Palestinians, after the Israeli prime minister said an offensive would go forward regardless of the outcome of truce talks with Hamas.

A military assault on Rafah would “be an unbearable escalation, killing thousands more civilians and forcing hundreds of thousands to flee,” he told reporters.

He added that while there has been “incremental progress” towards averting “an entirely preventable, human-made famine” in northern Gaza, much more is urgently needed, including for Israel to follow through on its promise to open “two crossing points between Israel and northern Gaza, so that aid can be brought into Gaza from Ashdod port and Jordan”.

He cited the lack of security for aid workers and civilians as the major obstacle to distributing aid across Gaza. “I again call on the Israeli authorities to allow and facilitate safe, rapid and unimpeded access for humanitarian aid and humanitarian workers, including UNRWA, throughout Gaza,” he said.

UNRWA Commissioner-General Philippe Lazzarini told journalists on Tuesday that aid supplies into Gaza had improved in April, but listed a series of continuing difficulties including regular crossing closures “because they [Israel] are dumping released detainees or dumping sometimes bodies taken to Israel and back to the Gaza Strip.”

Juliette Touma, a spokesperson for the UN’s refugee agency for Palestinians, said that Israel had sent 225 bodies to Gaza in three containers since December that were then transported by the UN agency to local health authorities for burial, shutting the crossing temporarily.

She did not have details of the circumstances of their deaths and said it was not UNRWA’s mandate to investigate.

Palestinian authorities have previously said that Israel has returned bodies after confirming they were not hostages. They said they were trying to identify them and figure out where they were killed.

Tensions are high between Israel and UNRWA, with the former accusing 19 UNRWA staff of involvement in the October 7 Hamas-led attack against Israel that killed at least 1,139 people, according to an Al Jazeera tally based on Israeli statistics.

Israel’s allegations are being examined by UN investigators although a separate review found Israel has yet to provide evidence for accusations that hundreds of UNRWA staff are members of armed groups.

More than 34,000 people have been killed in Israel’s war on Gaza since October 7, according to Palestinian authorities.