UN aid agencies say they have begun to significantly reduce their operations in the Gaza Strip because they have almost exhausted their fuel reserves.

Small quantities of fuel retrieved from existing reserves are being used to maintain the water supply in the south, where hundreds of thousands of people are sheltering from Israeli strikes.

However, they will run out on Thursday.

The agencies say they have reduced their support for overwhelmed hospitals and bakeries feeding the displaced.

"What we are seeing in the Gaza Strip is unprecedented," Juliette Touma of the UN agency for Palestinian refugees, Unrwa, told the BBC.

"Two million people are being strangled. Gaza is being choked with very, very little assistance that is coming from outside."

Israel began its bombing campaign in Gaza, cut off electricity and most water, and stopped imports of food, fuel and other goods in retaliation for a cross-border attack by Hamas on 7 October, in which at least 1,400 people were killed and 224 taken hostage.

Gaza's Hamas-run health ministry says 7,000 people have been killed in the territory since then and that its health system is facing total collapse, with a third of hospitals not functioning and the rest only treating emergency cases.

At least 74 lorries carrying food, water and medical supplies have crossed from Egypt via the Rafah border crossing since Saturday, which Ms Touma called "a drop in the ocean of overwhelming needs". About 500 lorries were allowed into Gaza every day before the start of the war.

Israel refuses to allow deliveries of fuel because it says it could be used for military purposes by Hamas, which it classes as a terrorist organisation - as do the UK, US and other powers.

But Ms Touma said Unrwa urgently needed fuel if it was to continue to serve as a lifeline for the 629,000 displaced people taking refuge inside its facilities. Most fled homes in the north of Gaza after being told by the Israeli military to leave for their own safety.

"We're the largest humanitarian organisation and we are on the verge of stopping operations. We are being banned from undertaking the mandate that was entrusted to us by the UN General Assembly. All we're asking to do is to be able to do our work," she added.

Dr Abdelkader Hammad, a surgeon at the Royal Liverpool University Hospital in the UK who arrived in Gaza to carry out transplants the day before the war began, is sheltering at a UN facility in the southern city of Rafah.

"The situation on the ground is deteriorating day by day," he told the BBC.

"People are fighting for the water, for the food, because of the shortages. Whatever they have in the stores is running out now. People are queuing for hours and hours to get some bread from the bakeries which are still working."

He also expressed alarm about the conditions at the hospitals where he usually performs operations, saying that his colleagues working at them were describing a "medical disaster".

"Theatres are full of wounded people. They have to make very difficult decisions about who they treat because they cannot cope with the sheer number of [wounded] people coming," he said. "They are running out of medical equipment. The fuel is running out."

But he advised Unrwa to ask Hamas to hand over some of the hundreds of thousands of litres of fuel that the military claims is being stored in a dozen tanks near the border with Egypt.

"There is enough for many days for hospitals and water pumps to run," he added. "Only the priorities are different. Hamas prefers to have all of the fuel for its war-fighting capabilities, leaving civilians without it."

Asked to comment on the allegation, the UN's regional humanitarian chief, Lynn Hastings, told the BBC: "We don't have any information about other fuel being available for Hamas to access."

"I know that is a concern of the Israelis, and quite rightly. It is something that we are trying to address with the Israelis to be able to bring in enough fuel for the humanitarian operations."

Ms Hastings warned that without fuel for their back-up generators Gaza's hospitals would no longer be able to care for about 1,000 patients receiving kidney dialysis treatment, 130 premature babies in incubators, and intensive care patients on ventilators.

Gaza's water desalinisation and pumping stations would also cease to function, she said.

"There is very, very little clean drinking water available now, which means people are resorting to drinking dirty or salinated water, or both."

"It also means that the sanitation system is blocked up, because there is no electricity to pump sewage into the sea. We are expecting the streets to have sewage overflow onto them imminently."

Ms Hastings also complained in a separate statement that the Israeli military was continuing to warn people in Gaza City to evacuate when they had nowhere to go or were unable to move.

"When the evacuation routes are bombed, when people north as well as south are caught up in hostilities, when the essentials for survival are lacking, and when there are no assurances for return, people are left with nothing but impossible choices," she warned.

In a statement, the Israeli military accused Hamas of using civilians as human shields, and stopping them from evacuating south.

"As we have seen in the past, they use a variety of methods including roadblocks," it said.