Thousands of people in Gaza have broken into aid depots to take supplies in what the UN called a "worrying sign of civil order starting to break down".
The UN relief agency for Palestinians (Unrwa) said warehouses holding basics like flour and soap were ransacked.
The news comes as Israel intensifies military action against Hamas in Gaza.
More than half of the Strip's 2.4 million inhabitants have been displaced and thousands of buildings have been destroyed, the UN says.
Hamas gunmen launched unprecedented attacks on Israel from Gaza on 7 October, killing more than 1,400 people and taking 230 hostages.
Since then, Israel has been carrying out strikes and now ground operations in Gaza, where the Hamas-run Palestinian health ministry says more than 8,000 people have been killed.
'People are desperate'
Unwra said its second largest depot - located in Deir al-Balah, in the central Gaza Strip - was targeted on Saturday as it holds most of the humanitarian aid delivered by the UN.
People took flour, wheat and hygiene kits after storming this and several other storage facilities in the centre and south of Gaza, the agency said.
A senior spokeswoman for the UN World Food Programme (WFP), Abeer Etefa, told the BBC that the raids on warehouses were "expected" because of the "difficult conditions facing people".
"The bottom line is that people are desperate, they are hungry," she said.
Some of the displaced Gazans who took items from the UN warehouses told AFP news agency they had "no flour, no aid, no water, not even toilets".
Gaza resident Abdulrahman al-Kilani said: "Our houses were destroyed. No one cares about us. We appeal to the people of the world. All international powers are against us. We needed aid, and we wouldn't have done this if we weren't in need."
Very little aid, and no fuel, has been allowed to enter Gaza since the attack by Hamas. Israel says it is concerned that supplies will be used by Hamas to continue the conflict.
But officials on Sunday indicated that Israel would allow more aid into the strip in the coming days.
In a video statement posted on social media, Israel Defense Forces (IDF) spokesman Rear Admiral Daniel Hagari said humanitarian aid from Egypt and the US would be expanded into Gaza on Sunday, without elaborating further on the details.
However spokesman for Egypt's foreign ministry, Ahmed Abu Zeid, accused Israel of "obstructing" aid deliveries by implementing cumbersome security checks.
Meanwhile, UN Secretary-General António Guterres said the situation in Gaza was growing more desperate by the hour.
"More than two million people, with nowhere safe to go, are being denied the essentials for life - food, water, shelter and medical care - while being subjected to relentless bombardment. I urge all those with responsibility to step back from the brink," the UN chief said.
