Gaza City, Gaza - A UN agency warned dwindling fuel supplies could force it to stop aid operations in war-riven Gaza on Wednesday, as calls mounted for a humanitarian "pause" in fighting.

After 18 days of withering Israeli air strikes and a near-total land, sea, and air blockade of the Palestinian territory, the UN agency for Palestinian refugees (UNRWA) warned operations were at breaking point.

"If we do not get fuel urgently, we will be forced to halt our operations in the Gaza Strip," said the UN agency, which provides aid to 600,000 displaced Gazans.

Israel launched its assault on Gaza in response to an unprecedented cross-border attack by Hamas militants on October 7, when gunmen from the Palestinian group poured into Israel from Gaza.

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But there is growing international unease about the impact of Israel's response to the attack, in which Hamas militants killed more than 1,400 people and took over 200 hostages.

Gaza's health ministry says 5,791 people have been killed in the war so far, more than 2,000 of them children.

Aid agencies report hospitals in Gaza are overwhelmed, generators lack fuel, and shelters are heaving under the weight of an estimated 1.4 million displaced – more than half the population.

Since the war began, a few dozen trucks with essential supplies have been allowed to cross the Egyptian border into Gaza, far fewer than needed, according to aid agencies.

The Palestine Red Crescent said on Tuesday it had received the fourth batch of humanitarian aid, consisting of eight trucks.

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The supplies have included medicines, food and water, but not fuel, which Israel fears could end up in Hamas's hands.

UN Secretary-General António Guterres said Gaza was now seeing "epic suffering", while imploring Israel to safeguard civilians.

"The relentless bombardment of Gaza by Israeli forces, the level of civilian casualties and the wholesale destruction of neighborhoods continue to mount and are deeply alarming," he told the UN Security Council in New York.

"I am deeply concerned about the clear violations of international humanitarian law that we are witnessing in Gaza," he said.

That warning drew an angry response from Israeli Foreign Minister Eli Cohen, who cited the graphic ways in which Israeli civilians had been killed by Hamas, and asked: "Mr. secretary-general, in what world do you live?"

Israel's ambassador to the UN, Gilad Erdan, later called on Guterres to resign immediately.

Israel and its allies have so far rebuffed calls for a ceasefire. US President Joe Biden has said that option can only be considered if and when Hamas releases the more than 200 hostages it is holding in Gaza.

But US officials have since indicated that a more limited "humanitarian pause" in certain areas could help ease civilian suffering.

Secretary of State Antony Blinken said: "Food, water, medicine and other essential humanitarian assistance must be able to flow into Gaza."

"It means civilians must be able to get out of harm's way. It means humanitarian pauses must be considered for these purposes."

His comments came as Israel continued to mass tens of thousands of troops around Gaza ahead of an expected ground offensive.

That operation appears to have been stalled by concerns about objectives, the fate of hostages and civilians, and the difficulties of fighting in a densely populated area latticed by underground tunnels.