The UNRWA has been sharing its stockpile so trucks can distribute aid, bakeries can feed people in shelters, and water can be desalinated.

Fuel has also been sent to hospitals so incubators, life support machines and other essential equipment can continue to operate.

But relief workers have warned that remaining supplies will run out today if the agency continues to do all these things.

UNRWA spokeswoman Tamara Alrifai said: "Do we give for the incubators or the bakeries? Do we bump clean water or do we send trucks to the borders? It is an excruciating decision."

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The warning has sparked fears that the humanitarian crisis could worsen rapidly - with food, water and medicine also running low.

While a small number of trucks with aid have entered from Egypt, about 1.4 million of Gaza's 2.3 million residents have been displaced after fleeing their homes.

Israel has also barred fuel from making its way across the Rafah border crossing because it believes these supplies would be stolen by Hamas fighters.

Shortages are being keenly felt in Gaza's hospitals, where a lack of medicine and clean water have led to "alarming" infection rates, the group Doctors Without Borders said.

Doctors have had to perform amputations to stop infections from spreading further, with patients only receiving "slight sedation", the group added.

Save The Children is calling for an urgent humanitarian ceasefire so aid can get through, amid reports that people in Gaza are resorting to drinking sea water because desalination plants have no fuel.

"That will undoubtedly trigger deadly outbreaks of cholera and other waterborne diseases," spokeswoman Alison Griffin told Sky News.

She said aid is currently queuing at the Rafah crossing, adding: "Getting those trucks in right now is the difference between life and death.

"Those trucks contain drinking water, they have medical supplies onboard, they've got dignity kits for women, kits for babies - but without a humanitarian ceasefire, there's no way we can get that aid into the hands of families who so desperately need it."

'A very serious situation'

When asked whether the UK would call for a ceasefire, Deputy Prime Minister Oliver Dowden told Sky News the government was seeking a "pause" in the conflict so aid can get through.

He said there is a "very serious situation" on the ground in Gaza right now, but stopped short of calling it a humanitarian crisis.

Mr Dowden said it is not a "reasonable position" to ask Israel for a complete cessation of hostilities following Hamas's attack.

"We shouldn't be saying to the Israel government 'you've got to stop action' in a way that wouldn't enable it to eliminate this threat," he added.

Israel's ambassador to the UK, Tzipi Hotovely, told Sky News: "If there is a humanitarian crisis, I blame Hamas for it. Israel allows medicine, food, water into the Gaza Strip."

She also claimed that financial support provided to Gaza in recent years had been misused by Hamas to create an underground tunnel city - describing it as a "war machine".

Speaking to Kay Burley, Ms Hotovely said: "Every tunnel cost $3m (£2.5m) - altogether, Israel exposed over 30 tunnels. $100m (£82m) in humanitarian aid could have been supplied to the Gaza people - but no, Hamas created this war machine, this horrific underground metro."

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350,000 remain in northern Gaza

In recent weeks, Israel has been urging Palestinians to leave northern Gaza and move south - warning all those who stay would be regarded as possible "accomplices" of Hamas.

But some estimates suggest that about 350,000 Palestinians remain there - despite facing grave danger.

Of those, about 30,000 have decided to return north because southern Gaza is coming under bombardment - with overcrowded shelters and shortages of food and water.

Mahmoud Shalabi, one of those who decided not to evacuate, would only be prepared to leave if southern parts of the Gaza Strip were no longer under attack.

He said: "It doesn't make sense to me that I should leave my home to go and get killed in a tent in the south of Gaza."

Those who have stayed put are now living among the ruins of once-bustling neighbourhoods - relying on tins of beans, pineapple and corn because shop shelves are empty.

Mr Shalabi described walking for two hours in an attempt to find a shop selling bread to feed his family of 10 - and failing to find a functioning ATM among blocks of dilapidated buildings after running out of cash.