Even in Rafah, the densely populated area in southern Gaza with the greatest access to food, 10 percent of children under 2 are acutely malnourished, and 4 percent are severely wasting.

Before the war, UNICEF said, the rate of acute malnutrition among young children was less than 1 percent, and severe wasting was extremely rare.

Lucia Elmi, UNICEF’s special representative in the Palestinian territories, who returned from Gaza last week, said she was particularly alarmed by not only the number of children suffering from malnutrition, but how quickly their health was deteriorating. Young children cannot be adequately nourished from just water, flour and bread, she said.

“They need protein, they need vitamins, they need fresh products and they need micronutrients, and all of this has been completely missing,” Ms. Elmi said in an interview last week. “That’s why the deterioration has been so fast, so rapid and at this scale.”

Children are bearing extreme costs of the war in Gaza, both physically and mentally, children’s rights groups and experts have repeated. More than 12,000 children have been killed in the conflict, and 27 children in northern Gaza have died from malnutrition or dehydration, according to the Gazan Health Ministry.

Palestinian parents say that, in addition to the threat of bombardment, their daily struggle is to find enough food for their children. Many have said they choose to feed what little they have to their children rather than themselves.

Dominic Allen, the United Nations Population Fund representative for Palestine, who just returned from a trip to Gaza, said on Friday that conditions there were worse than he could “describe or than pictures can show or than you can imagine.” He said at a press briefing in Jerusalem that everyone he saw or spoke to was “gaunt, emaciated, hungry.”

“The situation is beyond catastrophic,” he said.

Israel has said that it does not limit the amount of aid allowed into Gaza through border crossings, and recently signaled its support for new initiatives to get aid into Gaza by land, air and sea. Humanitarian groups have criticized Israel, saying that its insistence on checking every truckload of aid — and rejecting some — is a major cause of the food shortage.

The chief executive of Save the Children, an aid group, in the United States, Janti Soeripto, said that the crisis was currently, by far, the worst in the world for children.

“Every time I speak about Gaza, I sort of think to myself that it couldn’t get any worse,” she said in an interview. “And then every week, I’m proven wrong.”

Without a cease-fire, it has been difficult for teams to safely and comprehensively assist Palestinians.

Speaking from Rafah, Rachael Cummings, Save the Children’s director of humanitarian public health in the United Kingdom, said that the lack of sanitation — including dirty or salty water and sewage on the streets — was worsening the hunger crisis there.

“If a child isn’t eating adequate food or the right composition of food — they have poor water, poor sanitation — they will get very sick, very quickly,” she said.