Aid agencies say the already fragile humanitarian situation in the Gaza Strip has deteriorated dramatically since Israel began its military offensive.
Israel has imposed a crippling blockade on Gaza for the past 18 months, allowing little more than humanitarian basics into the coastal territory.
Israel says it is working with agencies to solve the humanitarian problems
Health, energy and water infrastructure were already close to
breaking point before the fighting broke out.
Israel has stopped maintaining, as it did for the first week of the operation, that there is "no humanitarian crisis" in the territory.
It now says it is working with international organizations to solve humanitarian problems.
But it says Hamas is responsible for the humanitarian situation in Gaza, and is "holding the people hostage" by targeting Israeli civilians.
On Monday, UN humanitarian coordinator Max Gaylard said: "Large numbers of people, including many children, are hungry, they are cold and without ready access to medical facilities, without access to electricity and running water, above all they are terrified - that by any measure is a humanitarian crisis."
Save the Children says there is a "severe shortage of food", but Israel claims international agencies' warehouses are well stocked and points out that it has allowed several convoys of trucks into Gaza during the fighting.
The UN says 23 of Gaza's 47 bakeries have been forced to close
Some 750,000 people - half Gaza's population - are dependent on food hand-outs from the UN relief agency, Unrwa. In December, it had to suspend distribution at times because it ran out of flour after Israel closed the border crossings into Gaza repeatedly.
On Monday it said it had two days' worth of flour stocks, although another delivery was expected. Unrwa stressed that although food supplies were coming in on trucks, the necessary volumes could not be shipped unless the conveyor belt for grain at the Karni crossing - close to the scene of recent heavy ground clashes - was re-opened.
Unrwa said the fighting, and the Israeli military blocking the main north-south road were hampering distribution, and that two of its distribution centres had been forced to close on Sunday.
Another agency, the World Food Programme, has 3,800 tonnes of food in Gaza, but much of this is in warehouses near the Karni crossing, also inaccessible because of the fighting. More is in the warehouses of the Palestinian interior ministry, whose workers are too afraid of the violence to turn up for work, it says.
The UN says 23 of Gaza's 47 bakeries have been forced to close due to shortages of cooking gas, and that another 14 are operating sporadically.
In addition, Israel's destruction of smuggling tunnels, used to bring in both weapons and other products, has disrupted the flow of other food items.
Prices for many foodstuffs have risen.
And, because of the long term economic impact of the blockade, plus shortages of bank notes, many Gazans cannot afford to buy much in the first place.
Hospitals have been under extreme pressure, with some 2,500 people wounded in the fighting.
Emergency workers are struggling to reach the wounded and a number are reported to have died trying.
Basically people are dying while they're waiting
The International Committee of the Red Cross (ICRC) has been attempting to co-ordinate safe passage with the Israelis, but says in many cases this has not been possible or has taken too long.
"Basically people are dying while they're waiting," said spokeswoman Sophie-Anne Bonefeld.
An ICRC team of specialist medical personnel, led by a war surgeon, waited for nearly four days before being given access to Gaza, while doctors in the territory's hospitals have been working around the clock.
Medical supplies are entering the territory on the trucks Israel has allowed in, but aid agencies say there are difficulties getting the correct items to the places where they are needed.
Gaza holds 1.5m inhabitants, more than half of them children
And some urgently needed items remain unavailable - the ICRC, for example, says it is trying to get 1,000 doses of tetanus vaccine through the crossings because, according to its information, there is none left in Gaza.
Five of Unrwa's 18 medical clinics have closed because of the fighting.
The UN said on Sunday that all of Gaza City's hospitals had been without mains electricity for 48 hours.
They are depending on generators designed only to work as back-ups, and have suffered from shortages of spare parts due to the blockade.
In Gaza City's biggest hospital, al-Shifa, the lives of some 70 intensive-care patients hooked up to machines are dependent on the generators - and the UN says the hospital has only three days' fuel supply to power them.
The UN says a million people in Gaza are without electricity. The territory's only power plant, which supplies much of Gaza City, shut down on 30 December because it ran out of industrial diesel fuel.
Gaza's power plant is frequently forced to shut down due to fuel shortages
A delivery of 215,000 litres of industrial diesel were transferred into Gaza on 5 January - about 10% of what the Supreme Court has set as a minimum weekly level to be allowed through under the blockade.
Another 100,000 litres of regular diesel were also transferred, but on Monday night neither delivery had been picked up on the Palestinian side because secure passage had not been agreed for the workers transporting it, the Israeli NGO Gisha said.
There are also severe shortages of cooking gas.
Israel says the one crossing through which fuel can be transported - Nahal Oz - was closed for the first week of the operation "due to security concerns".
The UN estimated on 5 January that 250,000 people did not have access to running water.
Many of Gaza's wells rely on pumps normally powered with electricity that are now dependent on generators, and fuel is running out.
Many wells are not working at all due to lack of power and damaged pipes
According to Gaza's water utility body, 48 of the territory's 130 wells are not working at all due to lack of power and damage to the pipes, while another 45 are operating partially.
The ICRC says half a million people in Gaza City may be without water within 48 hours as fuel for the pumps runs out.
Gaza's water utility body also warned on Sunday that sewage was flowing into populated areas, farmland and the sea, as five of Gaza's 37 wastewater pumping stations had shut down because of power cuts.
The rest would shut down within three or four days unless diesel is delivered, it said.
Even if diesel enters Gaza, transporting it safely to where it is needed may be impossible because of the fighting.The authority also fears that shelling may damage a large sewage lagoon in the north of the territory, and flood the urban area of Beit Lahiya.
Designed and developed by : Mohamed Hamdy