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UN Secretary General Antonio Guterres on Monday called for an independent investigation into the killing and wounding of Palestinians near an aid distribution point run by the US and Israel-backed Gaza Humanitarian Foundation (GHF).
Gaza's civil defence agency said at least 31 Palestinians died and more than 170 were injured after Israeli troops opened fire at the distribution site in the southern city of Rafah, with medics and witnesses corroborating the reports.
Israel's army first said it was unaware of casualties, then said its troops did not fire at civilians "near or within" the food bank in the south of Gaza, adding that "reports to this effect are false".
Mr Guterres did not assign blame for the deaths.
"I am appalled by the reports of Palestinians killed and injured while seeking aid in Gaza yesterday. It is unacceptable that Palestinians are risking their lives for food," the UN chief said.
"I call for an immediate and independent investigation into these events and for perpetrators to be held accountable."
Israel's Foreign Ministry called the statement by the UN chief a "disgrace" and criticised him for not mentioning Hamas.
Another three Palestinians were killed early on Monday near an aid centre west of Rafah in the south, medical sources told The National.
The UN has criticised the aid distribution plan, which leaves the usual co-ordinators such as Palestinian refugee agency UNRWA out of the loop. Little is known about the US-backed Gaza Humanitarian Foundation, and aid groups say it endangers rather than helps civilians by delivering food through narrow, militarised corridors.
"Israel has clear obligations under international humanitarian law to agree to and facilitate humanitarian aid," Mr Guterres added.
Gazans have accused Israel of engineering a humanitarian catastrophe with its new aid operations, saying supplies are being restricted to the south of the enclave in an effort to displace people from the north through starvation.
Israel has faced mounting international criticism over the humanitarian crisis in Gaza, where the UN has warned the entire population faces famine. It imposed an aid blockade on the besieged strip in March and has only relaxed it in recent days.
Aid is now trickling in after Israel partially lifted its blockade, but the UN has reported looting of its lorries and warehouses.
The wrangling over aid comes as US President Donald Trump's special envoy Steve Witkoff on Saturday said Hamas had submitted a “totally unacceptable” response to a US-backed ceasefire plan signed off by Israel.
The 60-day truce proposal was presented to Hamas on Thursday and now appears to be in the balance.
Hamas had given a conditional agreement to the plan, sources told The National, with the group's reservations focused on assurances it seeks on Israel's withdrawal from the Palestinian territory and the distribution of aid.