Gaza - United Nations chief Antonio Guterres on Tuesday urged Israel not to invade southern Gaza's Rafah city after the Israeli prime minister said an offensive would go forward regardless of a pending hostage deal with Hamas.

With the war roiling the region, international outrage mounting over the human toll, and mediators ramping up diplomatic efforts to ease the crisis and reach a truce, Guterres implored Israel not to go ahead with its operation.

"A military assault on Rafah would be an unbearable escalation, killing thousands more civilians and forcing hundreds of thousands to flee," the secretary-general told reporters.

Such an operation "would have a devastating impact on Palestinians in Gaza, with serious repercussions on the occupied West Bank and across the wider region," he added.

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"All members of the Security Council, and many other governments, have clearly expressed their opposition to such an operation. I appeal for all those with influence over Israel to do everything in their power to prevent it."

The latest Guterres warning follows Prime Minister Benjamin Netanyahu's promise that the Israeli army will launch a ground offensive on the Gaza Strip's far-southern city of Rafah "with or without" a truce deal being agreed with Hamas.

Rafah has become a refuge for some 1.5 million Palestinians who have fled Israel's bombardments that have ravaged the territory since the start of the war on October 7, when Hamas launched a deadly and unprecedented attack on Israel.

But Netanyahu has pledged to destroy Hamas, and he said the Israeli army stopping the war "before achieving all of its goals is out of the question."

"We will enter Rafah, and we will eliminate the Hamas battalions there with or without a deal," he told families of hostages still being held in Gaza, his office said.

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Guterres implored Israel and the Palestinian Islamist movement Hamas to reach a truce "now," and reiterated his call for Israel to allow more humanitarian aid into Gaza, including through the UN agency for Palestinian refugees, or UNRWA.

"Without that, I fear the war, with all its consequences both in Gaza and across the region, will worsen exponentially," he said.

The secretary-general also expressed alarm Tuesday over reports of mass graves discovered at Gaza's two main hospitals, along with allegations those buried there were unlawfully killed.

"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," he said.