The United Nations World Food Programme said "hordes of hungry people" broke into one of the agency's warehouses in central Gaza on Wednesday and initial reports indicated that two people had died and several more were injured.
"Gaza needs an immediate scale-up of food assistance. This is the only way to reassure people that they will not starve," WFP said in a statement.
Additionally, Gaza residents and merchants said that UN trucks delivering food to Gaza were stopped and looted overnight.
These events come after desperate Palestinians overran a distribution site run by a US-backed group trying to start delivering aid.
The incidents underscore the problems getting supplies to hundreds of thousands of Palestinians facing worsening hunger and starvation after a weeks-long Israeli blockade.
On Tuesday, Israeli troops fired warning shots as crowds rushed to a distribution point run by the Gaza Humanitarian Foundation, a US-backed group that began supplying aid under a new system which Israel hopes will prevent aid from reaching Hamas.
The United Nations and other international aid groups have refused to take part, saying the scheme violates the principle that aid should be distributed neutrally, based only on need.
New aid systems begin in Gaza Strip
As the new system began, the Israeli military also allowed 95 trucks belonging to the UN and other aid groups into the enclave, but three Gaza residents and three merchants said a number of trucks were targeted by looters.
One Palestinian transport operator said at least 20 trucks belonging to the UN World Food Programme were attacked shortly before midnight.
"Some trucks made it through, then it seems that people became aware of that," one witness told Reuters via a chat app, declining to be named because of the sensitivity of the issue.
"They woke up, some placed barriers on the road, intercepted, and stole the goods."
Israeli forces, which resumed their operation in Gaza in March following a brief truce, continued strikes on Wednesday, killing at least 15 people, including eight members of the family of a local journalist, Palestinian health officials said.
To qualify for aid under the new system, people seeking food are supposed to undergo screening to ensure they are not linked to Hamas, a measure that has heightened Palestinian suspicion of the operation.
But witnesses on Tuesday said that no effective identification process seemed to be in place.
"What we saw yesterday was a very clear example of the dangers of distributing food," said Ajith Sunghay, Head of the UN Office of the High Commissioner for Human Rights in the Occupied Palestinian Territory.
"We are exposing people to death and injury," he told reporters in Geneva, adding that 47 people had been wounded by gunfire as the chaos unfolded.
Footage shared on social media showed fences broken down by crowds trying to reach crates of supplies as private security contractors operating the site fell back.
"I am a big man, but I couldn't hold back my tears when I saw the images of women, men, and children racing for some food," said Rabah Rezik, 65, a father of seven from Gaza City.
Israel imposed the blockade on aid supplies in March, accusing Hamas of seizing supplies meant for civilians, a charge Hamas denies. UN officials say they have seen no evidence that the militant group has been looting trucks since Israel eased the blockade this month under mounting international pressure.
US slams lack of UN participation
However, Hamas has told people in Gaza not to go to the four distribution points in southern Gaza set up for the new system. It denied accusations from Israel that it was blocking access to the sites.
The US ambassador to Israel, Mike Huckabee, called it "sad and disgusting" that the United Nations and other groups were not taking part in the new system to distribute aid.
"There were lines of people that got food, which was not stolen by Hamas. The manner in which it was distributed is effective so far," he told Reuters.
Israel has faced increasing pressure over the dire humanitarian situation in Gaza, even from countries long reluctant to voice strong criticism. France, Britain, and Germany have said they may take action if the military campaign is not halted. On Wednesday, Italy also said the offensive had become unacceptable and must stop immediately.
Israel launched its campaign in Gaza in retaliation for the Hamas-led attack on communities in southern Israel on October 7, 2023, that killed some 1,200 people and saw 251 hostages abducted into Gaza.
Its assault has killed more than 54,000 Palestinians and reduced much of the crowded coastal enclave to rubble, with the population of more than 2 million now squeezed into narrow areas on the coast and around the southern city of Khan Younis.