Palestinians displaced by the Gaza war are living in "appalling" conditions, with children sometimes going for a whole day without food and thousands sharing the same toilet, Oxfam warned on Tuesday.
Deadly Israeli bombardment and fighting has raged in the Gaza Strip's far-southern Rafah area near the Egyptian border in recent weeks, again displacing those who had fled there in search of safety.
More than one million people have fled Rafah for other areas, according to the UN Palestinian refugee agency, UNRWA.
Oxfam said more than two-thirds of Gaza's population is estimated to be crammed into less than a fifth of the besieged territory.
"Despite Israeli assurances that full support would be provided for people fleeing, most of Gaza has been deprived of humanitarian aid, as famine inches closer," the aid agency said.
"A food survey by aid agencies in May found that 85 percent of children did not eat for a whole day at least once in the three days before the survey was conducted," it added.
Since Israeli troops launched their ground assault on Rafah on May 6, an average of eight aid trucks per day have entered, Oxfam said, citing UN figures.
While hundreds of commercial food trucks are estimated to be entering daily, the goods on board include non-nutritious energy drinks, chocolate and cookies, and are often very expensive, it added.
"By the time a famine is declared, it will be too late," Oxfam's Middle East and North Africa director, Sally Abi Khalil, said.
"Obstructing tonnes of food for a malnourished population while waving through caffeine-laced drinks and chocolate is sickening."
In an interview with French television last week, Israeli Prime Minister Benjamin Netanyahu rejected allegations of starvation in Gaza, saying "everything had been done" to avert a famine.
Oxfam said families in some parts of southern Gaza, like the coastal area of Al-Mawasi, designated a "humanitarian zone" by the Israeli army, were getting by with barely any water or sanitation services.
"Living conditions are so appalling that in Al-Mawasi, there are just 121 latrines for over 500,000 people -- or 4,130 people having to share each toilet," Oxfam said.
Meera, an Oxfam staff member in Al-Mawasi who has been displaced seven times since October, described conditions there as "unbearable".
"There is no access to clean water, and people are forced to rely on the sea," she said.
On Monday, sewage flooded a camp for the displaced in Khan Younis after a wastewater pipe burst, an AFP reporter said, with some trying to scoop the filth out of their tents using plastic bottles.
The war was sparked by a Hamas-led attack in southern Israel on October 7, which resulted in the deaths of around 1,170 people, according to an AFP tally based on Israeli official figures. Gunmen also took around 250 hostages, 120 of whom remain in Gaza.
Hamas says the attack came in response to Israel's decades-old occupation of Palestine and continued aggression against the Palestinian people.
Israel's unprecedented offensive has killed at least 36,550 people in Gaza, mostly women and children, according to the territory's health ministry. Thousands more are believed to be buried beneath the rubble.